Clinical Pathology: Clinical Chemistry PDF

SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE Clinical Pathology: Clinical Chemistry 1...

SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE Clinical Pathology: Clinical Chemistry 1 CH A P TE R Patricia A. Schenck 2. Better method than endogenous creatinine EVALUATION OF RENAL FUNCTION clearance and approximate inulin clearance I. Blood urea nitrogen (BUN) in dogs A. Decreased glomerular filtration rate (GFR) results C. Single-injection methods for estimation of GFR in increased BUN 1. Post-iohexol clearance B. Affected by urea production in the liver and the a. Give iohexol IV, and collect plasma at rate of excretion by the kidney 2, 3, and 4 hours postiohexol C. Increased dietary protein and gastrointestinal b. Plasma clearance is calculated using the area (GI) bleeding both increase BUN under the plasma concentration vs. time curve II. Creatinine 2. Inulin clearance is considered the gold stan- A. An elevation indicates that less than 25% of the dard for measurement of GFR, but inulin is original functioning renal mass remains not easily measured and is not available at B. A normal serum creatinine concentration does commercial laboratories not exclude the possibility of renal disease V. Urine osmolality C. Young animals have lower serum creatinine A. There is usually a linear relationship between concentrations than do older animals urine osmolality and specific gravity D. Cachexia often causes lower serum creatinine B. Urine osmolality depends on the number of concentrations osmotically active particles present in urine III. Serum phosphorus concentration C. If urine contains a large amount of larger-molecular- A. An increase in serum phosphorus is not seen until weight solutes such as glucose, mannitol, or radio- more than 85% of nephrons are nonfunctional in graphic contrast agents, the urinary specific gravity chronic renal diseases will be disproportionately high compared with the B. Tubular reabsorption of phosphorus is regulated osmolality by parathyroid hormone. Renal secondary hyper- VI. Fractional excretion of electrolytes parathyroidism tends to keep the serum phos- A. Sodium phorus concentration within normal limits by 1. Useful in the differentiation of prerenal and excreting more phosphorus into the urine until primary renal azotemia renal disease is advanced 2. In animals with prerenal azotemia and volume C. Serum phosphorus concentrations can be much depletion, there should be sodium conservation higher in immature animals because of bone with a very low fractional excretion of sodium growth 3. In animals with primary renal disease, the frac- IV. Renal clearance (estimation of GFR) tional excretion of sodium should be higher A. Endogenous creatinine clearance determination than normal 1. Collect all urine for 12 or 24 hours (record B. Potassium volume), and determine serum and urine 1. May be useful in the evaluation of chronic renal creatinine concentrations failure patients with hypokalemia to determine 2. Performed when renal disease is suspected but whether the kidneys are contributing to the both BUN and serum creatinine concentrations hypokalemia are normal 2. Varies considerably depending on diet B. Exogenous creatinine clearance C. Phosphorus 1. Creatinine is administered subcutaneously 1. May be useful during treatment of chronic or intravenously (IV); then urine is collected renal failure to determine whether dietary or via catheterization three times at 20-minute drug therapy is effective with a reduction in intervals fractional excretion of phosphorus 1 2 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE 2. Does not offer any advantage in diagnosis of acidosis, respiratory acidosis, or paradoxical chronic renal failure aciduria in metabolic alkalosis with potas- VII. Urinary enzymes sium and chloride depletion A. -glutamyltransferase (GGT) is a membrane- d. Causes of alkaline urine include plant-based bound enzyme specific for renal tubular damage diets, urine that has been allowed to B. N-acetyl--D-glucosaminidase (NAG) stand open to air at room temperature, 1. Lysosomal enzyme produced by many tissues postprandial alkaline tide, urinary tract but not filtered normally infection (UTI) by urease-positive organ- 2. Increases in urinary NAG are specific for renal isms, contamination of sample with bacte- tubular damage ria during or after collection, administra- VIII. Urinalysis tion of alkalinizing agents, metabolic A. Physical properties alkalosis, respiratory alkalosis, stress in- 1. Color duction of respiratory alkalosis (cats), and a. Normally colorless to deep amber in color distal renal tubular acidosis (if very concentrated). Deep amber color 2. Protein may also be due to bile pigments a. Trace to 1 protein is normal in urine with b. Red or reddish brown color is due to intact high USG red blood cells (RBCs), hemoglobin, or b. Dipstick methods are more sensitive to myoglobin albumin than globulins c. Dark brown to black is most likely due c. False positives occur in very alkaline urine or to the conversion of hemoglobin to in urine contaminated with benzylalkonium methemoglobin chloride d. Yellow-brown to yellow-green is due to d. Renal proteinuria may result from increased bilirubin glomerular filtration of protein, failure of e. Green color may be due to Pseudomonas tubular reabsorption of protein, tubular infection or to oxidation of bilirubin to secretion of protein, protein leakage from biliverdin damaged tubular cells, or renal parenchy- 2. Appearance mal inflammation a. Urine is normally clear in dogs but may be e. Persistent moderate or heavy proteinuria cloudy in about 20% in the absence of urine sediment abnormali- b. Cloudy urine usually contains increased ties suggests glomerular disease cells, crystals, mucus, or casts f. Active sediment with mild to moderate c. Horse urine is typically cloudy because of proteinuria suggests inflammatory renal mucus disease or lower urinary tract disease d. Rabbit urine is white and opaque because 3. Glucose of the high concentration of calcium a. Normally not present in dog and cat urine carbonate b. Glucose appears in urine if plasma glucose 3. Odor exceeds approximately 180 mg/dL in the a. The normal odor of urine is due to volatile dog and 300 mg/dL in the cat fatty acids c. Causes of glucosuria include diabetes melli- b. An ammonia odor is due to release of tus (most common), stress or excitement ammonia by urease-producing bacteria (especially in cats), chronically sick cats in 4. Urine specific gravity (USG) is the weight of the absence of hyperglycemia, renal tubular urine compared to that of distilled water disease, administration of glucose-containing a. USG estimated by dipstrip is NOT accurate. fluids, and severe urethral obstruction in USG should be estimated by refractometry. some cats Make sure the refractometer is temperature 4. Ketones compensated and has different scales for a. Not normally present in dog and cat urine different species b. Inadequate consumption of carbohydrates b. First-morning urine samples typically have or impaired utilization of carbohydrates can the highest urinary concentration lead to ketone production c. Dogs or cats with any detectable dehydration c. Causes of ketonuria include diabetic keto- should elaborate maximally concentrated acidosis (most common), starvation or urine (USG 1.040) prolonged fasting, glycogen storage dis- B. Chemical examination ease, low carbohydrate-high fat diet, and 1. pH persistent hypoglycemia (decreased insulin a. Measurement by pH meter is superior to induces ketone formation) dipstrip methods 5. Bilirubin b. Urine pH varies with diet and acid-base a. Only conjugated bilirubin appears in the balance. Urine pH is usually acidic in urine. A small amount of bilirubin may carnivores and alkaline in herbivores normally be seen in concentrated urine c. Causes of acidic urine include meat diets, samples from normal male dogs. It is not administration of acidifying agents, metabolic normally found in cat urine  CHAPTER 1 Clinical Pathology: Clinical Chemistry 3 b. Bilirubin is derived from the breakdown of (3) Clumps or “rafts” are most common heme by the reticuloendothelial system in neoplasia but may occur with c. Bilirubin may appear in the urine prior to inflammation the observation of hyperbilirubinemia c. Renal epithelial cells d. Causes of bilirubinuria include hemolysis, (1) Small epithelial cells from the renal liver disease, extrahepatic biliary obstruc- tubules or pelvis tion, fever, and starvation (2) Appearance in urine is never normal 6. Blood and is observed in patients with isch- a. Positive earlier than the observation of emic, nephrotoxic, or degenerative hematuria renal disease b. Dipstick tests do not differentiate from 5. Casts are cylindrical molds of the renal tubules intact RBCs or hemoglobin composed of aggregated protein or cells c. Causes of hemoglobinuria from hemolysis a. Hyaline include transfusion reaction, immune (1) Pure protein precipitates of Tamm- mediated hemolytic anemia, disseminated Horsfall mucoprotein intravascular coagulopathy (DIC), splenic (2) Dissolve rapidly in dilute or alkaline torsion, severe hypophosphatemia, heat urine stroke, zinc toxicity, and phosphofructoki- (3) Have the least pathologic significance nase or pyruvate kinase deficiency and may form transiently with fever, C. Urinary sediment exercise, or passive congestion to the 1. Sediment preparation kidney a. Perform on fresh urine samples b. Cellular casts b. Centrifuge 5 to 10 mL of urine at 1000 to (1) White cell casts suggest pyelonephritis 1500 rpm for 5 minutes. Stain with but may also be caused by interstitial ne- Sedi-Stain phritis, nephrosis, or glomerulonephritis c. Number of casts is recorded per low-power (2) Red cell casts are fragile and rarely field, and cells are recorded per high-power found. They may be noted in acute field glomerulonephritis, renal trauma, or 2. RBCs after violent exercise a. Occasional RBCs are normal (3) Hemoglobin casts are casts where the b. Excessive number of RBCs is called hema- hemoglobin color is retained in the cast turia, but origin cannot be determined (4) Renal epithelial casts occur with severe c. Lipid droplets are often confused with tubular injury and suggest acute tubular RBCs, especially in cats necrosis or pyelonephritis (Figure 1-1) d. Causes of hematuria include trauma, (5) Renal fragments are a variant of epithe- urolithiasis, neoplasia, UTIs idiopathic lial casts where portions of the renal feline lower urinary tract disease, chemi- tubules slough into urine. Their appear- cally induced cystitis, systemic diseases ance suggests severe renal injury associated with hemorrhage, renal infarct, (6) Mixed casts contain multiple cell types nephritis, nephrosis, parasites, renal c. Granular casts (Figure 1-2) pelvic hematoma, or genital tract (1) Represent the degeneration of cells or contamination precipitation of filtered plasma proteins 3. White blood cells (WBCs) (2) Fatty casts are a type of granular cast a. Occasional WBCs are normal that may be seen in nephrotic syn- b. Excessive WBCs in urine sediment is called drome or diabetes mellitus pyuria and indicates inflammation some- where in the urinary tract or contamination from the genital tract c. Clumped WBCs are typically due to infectious organisms d. Causes of pyuria include urinary tract inflammation or genital tract contamination 4. Epithelial cells a. Squamous epithelial cells (1) Large, polygonal cells with small, round nuclei (2) Common in voided or catheterized samples b. Transitional epithelial cells (1) A small number is normal Figure 1-1 Photomicrograph of an epithelial cell cast in urine. Small renal (2) Increased in infection, irritation, or epithelial cells can be identified in this case (white arrows). (Courtesy Nancy neoplasia Facklam; from Ettinger SJ, Feldman EC. Textbook of Veterinary Internal Medicine, 6th ed. St Louis, 2005, Saunders.) 4 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE h. Urate crystals are associated with liver disease and portosystemic shunt i. Calcium oxalate crystals are found in acidic urine j. Cystine crystals are found in acidic urine and are associated with cystinuria k. Bilirubin crystals may be found normally in concentrated dog urine l. Oxalate monohydrate (“hippurate-like”) crystals are found in acute renal failure owing to ethylene glycol ingestion 8. Miscellaneous a. Sperm is common in urine from intact males b. Amorphous debris may be seen in those with acute intrinsic renal failure c. Mucous threads or fibrin strands may be seen in association with lower urinary tract or genital inflammation Figure 1-2 Photomicrograph of a finely granular case in urine. (From Ettinger SJ, Feldman EC. Textbook of Veterinary Internal Medicine, d. Parasite ova from Dioctophyma renale or 6th ed. St Louis, 2005, Saunders.) Capillaria plica are rarely seen e. Lipid droplets are associated with cellular degeneration f. Foreign material may be present, especially d. Waxy casts represent the final stage of in voided samples degeneration of granular casts. They sug- g. Precipitates of urine stain may look like gest chronic intrarenal stasis and are found urinary crystals in advanced chronic renal disease e. Broad casts are wide casts that form in FLUIDS AND ACID-BASE METABOLISM collecting ducts or dilated distal nephron. They suggest severe intrarenal stasis and I. Dehydration tubular obstruction A. Status 6. Organisms 1. Total body water is about 60% of body a. Normal urine is sterile weight; about half is extracellular and half is b. Large numbers of bacteria present (in associa- intracellular tion with pyuria) in urine collected by cathe- 2. Very mild dehydration occurs with water loss terization or cystocentesis strongly suggest of 1% to 4% of body weight. Clinical signs are the presence of UTI not detectable c. The presence of bacteria without pyuria 3. Mild dehydration occurs with water loss of should arouse suspicion for bacterial con- 5% to 6% of body weight. Clinical signs include tamination. However, dogs with hyperadre- dry mucous membranes, slight loss of skin nocorticism, diabetes mellitus, or immuno- turgor, injected conjunctiva, and inelasticity suppression and cats with chronic renal of skin disease can have bacteriuria with pyuria 4. Moderate dehydration occurs with water loss d. The absence of bacteria does not rule out of 7% to 9% of body weight. Clinical signs UTI include loss of skin turgor with slow return, e. Yeast and fungal hyphae in sediment are prolonged capillary refill time (2-3 seconds), usually contaminants enophthalmos 7. Crystals 5. Severe dehydration occurs with water loss a. Crystals are often an artifact of storage time of 10% to 12% of body weight. Clinical signs and refrigeration include extreme loss of skin turgor, peripheral b. Struvite crystals are found in alkaline urine vasoconstriction, cold extremities, and pro- and may be found in normal animals or in longed capillary refill time (3 seconds) those with struvite urinary stones 6. Very severe dehydration occurs with water loss c. Calcium phosphate crystals are found in of 13% to 15% of body weight; clinical signs in- alkaline urine clude vascular collapse, renal failure, and death d. Calcium carbonate crystals are found in B. Isotonic dehydration occurs with equal losses alkaline urine of water and solute e. Amorphous phosphate crystals are found 1. Sodium and chloride concentrations are not in alkaline urine affected f. Ammonium biurate crystals are found in 2. Increased packed cell volume (PCV) with alkaline urine increased plasma proteins g. Uric acid crystals are found in acidic urine 3. Occurs in some cases of diarrhea and renal and are associated with the Dalmatian breed disease  CHAPTER 1 Clinical Pathology: Clinical Chemistry 5 C. Hypertonic dehydration occurs when more water 2. Compensation is via a change in urinary acidi- than solute is lost fication to alter HCO3. This process is slower 1. Concentration of sodium and chloride increases than compensation in ventilation 2. PCV increases, with increased plasma proteins E. Simple acid-base disorders occur when there is a 3. Occurs most commonly in diabetes insipidus primary change, but no compensation has taken 4. Species that produce hypotonic sweat (cattle) place or little sweat (dogs, cats) develop hypertonic F. Compensated acid-base disorders occur when dehydration with heat stress primary changes are present, along with evidence D. Hypotonic dehydration occurs when more solute of a compensatory change in the complementary than water is lost system. The pH rarely returns to normal with 1. Concentrations of sodium and chloride compensation decrease G. Combined acid-base disorders occur when there 2. This results in a contraction of the extracellu- are changes in the same direction in both lar fluid (ECF) volume with expansion of intra- systems cellular fluid (ICF) volume to restore osmotic H. Metabolic acidosis equilibrium 1. Primary change is decreased HCO3 3. Most common type of dehydration, where the 2. PCO2 will decrease in compensation solute loss induces a secondary loss of water I. Metabolic alkalosis 4. Hypotonic dehydration from heat stress occurs 1. Primary change is increased HCO3 in species that produce hypertonic sweat 2. PCO2 will increase in compensation (horses) J. Respiratory acidosis II. Acid-base metabolism 1. Primary change is increased PCO2 A. Henderson-Hasselbach equation 2. HCO3 will increase in compensation 1. pH  pKa  log [A-]/[HA] 3. There is a larger compensation in chronic 2. The carbonic acid-bicarbonate system is usu- respiratory acidosis compared with an acute ally used: pH  pKa  log [HCO3-]/[H2CO3] event 3. pH  6.1  log[HCO3-]/0.03(PCO2) K. Respiratory alkalosis B. To characterize acid-base disorders, blood pH, 1. Primary change is decreased HCO3, and PCO2 are measured (Table 1-1) 2. PCO2 1. A decrease in pH is acidosis; an increase is 3. HCO3 will decrease in compensation alkalosis 4. There is a larger compensation in chronic 2. A decrease in HCO3 is a metabolic acidosis; an respiratory alkalosis compared with an acute increase is a metabolic alkalosis event 3. A decrease in PCO2 is a respiratory alkalosis; an L. Base excess and base deficit increase is a respiratory acidosis 1. Calculated from blood gas parameters by the 4. If HCO3 measurement is unavailable, total CO2 blood gas analyzer. This calculation is based from a chemistry profile can be used as an es- on human relationships and is probably valid timate. Total CO2 is about 1 to 2 mmol/L for dogs and cats. This calculation might not greater than the HCO3 concentration be valid for other species C. Metabolic disorders 2. Increased values reflect a base excess corre- 1. Characterized by changes in HCO3 sponding to metabolic alkalosis 2. Compensation is via rapid changes in ventila- 3. Decreased values reflect a base deficit, corre- tion to alter PCO2 sponding to metabolic acidosis D. Respiratory disorders M. Anion gap 1. Characterized by changes in PCO2 1. Anion gap  (Na  K) – (Cl  HCO3); the ob- jective is to estimate changes in the unmea- sured anions and cations without having to measure them a. Unmeasured anions include sulfate, Table 1-1 Characteristics of Primary Acid-Base lactate, phosphate, pyruvate, albumin, Disturbances and ketoacids b. Unmeasured cations include primarily pH [H] [HCO3] Pco2 calcium and magnesium 2. If the anion gap increases, then unmeasured Metabolic alkalosis ↑ ↓ ↑↑ ↑ anions have increased. If the anion gap Metabolic acidosis ↓ ↑ ↓↓ ↓ decreases, then unmeasured cations have Respiratory acidosis ↓ ↑ ↑ ↑↑ increased Respiratory alkalosis ↑ ↓ ↓ ↓↓ III. Osmolality GI loss (vomiting, A. Osmolality is the concentration or number of os- diarrhea) motically active particles in an aqueous solution B. Osmolal gap is the difference between the actual measured serum osmolality and the calculated es- Primary events are indicated by double arrows. timate of osmolality 6 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE 1. Calculated osmolality (mOsm/L)  1.86 [Na 3. Solute gain occurs with salt poisoning, hyper- (mmol/L)]  [glucose (mg/dL)/18]  [BUN tonic fluid administration, hyperadrenocorti- (mg/dL)/2.8]  9 cism, hyperaldosteronism 2. The osmolal gap increases when there is an in- II. Chloride crease in any osmotically active particles that A. Roles are not included in the calculated equation 1. Principal anion in ECF 3. The osmolal gap will increase whenever the 2. Chloride usually accompanies sodium to main- anion gap is increased tain neutrality 4. Used commonly in cases of ethylene glycol 3. Normal fractional excretion is less than 1% but toxicity may be elevated in large animals fed a diet a. Ethylene glycol is a small osmotically active higher in chlorine particle B. The same conditions causing hypernatremia b. The osmolal gap correlates well with the and hyponatremia also cause hyperchloremia concentration of ethylene glycol in serum and hypochloremia III. Potassium A. Roles ELECTROLYTE METABOLISM 1. Principal cation of the ICF I. Sodium 2. Determines resting cell membrane potential A. Roles B. Hypokalemia (typically associated with 1. Principal cation in ECF alkalosis) 2. Important in movement of fluids across epithe- 1. Decreased intake lial surfaces a. Anorexia B. Hyponatremia b. Dietary deficiency 1. Pseudohyponatremia c. Administration of potassium-free fluids a. Occurs with hyperlipidemia or hyperpro- 2. Translocation between ECF and ICF teinemia a. Metabolic alkalosis, respiratory alkalosis b. Plasma sample is diluted by the excess lipid b. Glucose or insulin administration or protein and thus the measured sodium c. Catecholamines concentration is falsely lowered 3. Increased loss 2. Hyperosmolal, hypervolemic conditions include a. GI loss hyperglycemia and mannitol administration (1) Vomiting 3. Hypoosmolal hypervolemic conditions (2) Diarrhea a. Occurs when there is excess water reten- b. Third-space syndrome tion with dilution of the plasma (1) GI obstruction (especially displaced b. Causes include nephrotic syndrome, abomasum) chronic liver disease, chronic renal failure, (2) Peritonitis and congestive heart failure (3) Ascites 4. Hypoosmolal euvolemic conditions include hy- c. Urinary loss potonic fluid infusion,antidiuretic hormone (1) Hyperadrenocorticism (ADH) administration, inappropriate secretion (2) Acute renal failure (nonoliguric) of ADH, and psychogenic polydipsia (3) Postobstructive diuresis 5. Hypoosmolal hypovolemic conditions include (4) Chronic renal failure (cats) the following: (5) Potassium-losing diuretics a. Dietary deficiency of sodium (6) Fanconi syndrome b. GI loss from vomiting or diarrhea (7) Renal tubular acidosis c. Third-space syndrome (GI obstruction, peri- (8) Primary hyperaldosteronism tonitis, uroabdomen, ascites) d. Cutaneous loss (burns) d. Urinary loss from hypoadrenocorticism, 4. Feline kaliopenic nephropathy-polymyopathy nonoliguric acute renal failure, diuretics, syndrome and Fanconi syndrome a. Characterized by hypokalemia, increased e. Cutaneous losses (burns) fractional excretion of potassium, azotemia, C. Hypernatremia and metabolic acidosis 1. Pure water deficits occur in dietary deficiency, b. Chronic decrease of potassium leads to de- central or nephrogenic diabetes insipidus, pri- crease in aldosterone, which leads to distal mary hypodipsia, heat stress, and fever renal tubular acidosis 2. Hypotonic fluid loss occurs with the following: C. Hyperkalemia (typically associated with a. GI loss owing to vomiting or diarrhea acidosis) b. Third-space syndrome (peritonitis, ascites) 1. Pseudohyperkalemia (in vitro translocation of c. Urinary loss from osmotic diuretics (manni- potassium to plasma) tol, diabetes mellitus), chronic renal failure, a. Thrombocytosis nonoliguric acute renal failure, postobstruc- b. Leukemia tive nephropathy c. Hemolysis (equine, bovids) d. Cutaneous loss (burns) d. Akita dogs  CHAPTER 1 Clinical Pathology: Clinical Chemistry 7 2. Increased intake or oversupplementation of B. Hypomagnesemia fluids with potassium 1. Most often occurs after excessive 3. Translocation between ICF and ECF magnesium loss a. Respiratory or metabolic acidosis a. GI tract (malabsorption syndromes, b. Hyperkalemic periodic paralysis diarrhea) c. Ischemia or reperfusion injury b. Renal loss (fluid diuresis, diuretic therapy, 4. Decreased urinary excretion renal disease) a. Anuric or oliguric renal failure 2. Iatrogenic deficiency occurs during fluid b. Urinary tract obstruction therapy as most fluids contain little or no c. Ruptured urinary bladder magnesium d. Hypoadrenocorticism 3. Metabolic disorders (diabetes mellitus, pri- e. Potassium-sparing diuretics mary hyperparathyroidism, primary hypopara- f. Nonsteroidal antiinflammatory drugs thyroidism, hyperaldosteronism, third-space (NSAIDs) syndrome, hypophosphatemia) g. Angiotensin-converting enzyme (ACE) 4. Ruminants inhibitors a. Milk tetany, in which calves are fed a IV. Phosphorus magnesium-deficient milk diet A. Roles b. Grass tetany, which occurs in adults fed on 1. Found mostly in ICF lush, green pasture that is high in potas- 2. Regulated through interactions with calcium sium, which blocks magnesium absorption and calcium metabolic hormones from the rumen a. Calcitriol increases phosphorus resorption C. Hypermagnesemia from bone, increases GI phosphorus absorp- 1. Renal disease (both acute and chronic) tion, and increases urinary fractional excre- 2. Increased renal tubular reabsorption of magne- tion of phosphorus sium during dehydration, salt depletion, and b. The concentrations of calcium and phos- hypoadrenocorticism phorus are reciprocally related and are 3. Overadministration of magnesium-containing kept relatively constant antacids B. Hypophosphatemia VI. Calcium 1. Increased cellular uptake of phosphorus A. Roles (glucose administration) 1. Major structural role in the skeleton 2. Acid-base balance 2. Important in regulation of ions across a. Respiratory alkalosis membranes b. Metabolic acidosis (enhanced urinary 3. Cofactor in many metabolic processes excretion of phosphates); often in diabetic 4. Major role in signal transmission, skeletal ketoacidosis muscle contraction, and cardiovascular 3. Abnormalities in renal tubular phosphate function reabsorption B. Measurement of calcium a. Hyperparathyroidism 1. Ionized calcium should be measured for an b. Fanconi syndrome accurate assessment of calcium status c. Aminoglycoside nephrotoxicosis 2. Adjustment formulas for total calcium should 4. GI absorption not be used because they do not reliably pre- a. Decreased phosphorus in diet dict ionized calcium concentration b. Vomiting 3. Acidosis increases ionized calcium concentra- c. Diarrhea tion, and alkalosis decreases ionized calcium d. Intestinal malabsorption syndromes concentration e. Excessive ingestion of phosphate binders C. Hypocalcemia C. Hyperphosphatemia 1. Primary hypoparathyroidism is characterized 1. Redistribution between ICF and ECF by hypocalcemia with a low or low-normal con- 2. Cellular damage centration of parathyroid hormone (an inap- 3. Acute acidosis (chronic metabolic acidosis propriate response). Hypomagnesemia may causes hypophosphatemia usually) also be seen. Primary hypoparathyroidism can 4. Decreased renal blood flow and GFR (resulting be spontaneously occurring; result from para- in secondary hyperparathyroidism) thyroiditis or parathyroid adenoma infarction; 5. Ruptured urinary bladder or occur postoperatively after removal of a 6. Hypertonic sodium phosphate enemas parathyroid adenoma or any other neck sur- 7. Excessive dietary intake (with secondary hy- gery that can interrupt the blood supply to the perparathyroidism) parathyroid glands V. Magnesium 2. Common causes of hypocalcemia include A. Roles hypoalbuminemia, chronic renal failure (ionized 1. Magnesium is an important cofactor for many hypocalcemia), eclampsia, acute renal failure, enzymatic reactions acute pancreatitis, and urethral obstruction 2. Influences cell membrane properties in cats 8 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE 3. Occasional causes of hypocalcemia include EVALUATION OF THE LIVER soft tissue trauma, rhabdomyolysis, ethylene glycol poisoning, phosphate enemas, post I. Enzymes in the liver bicarbonate administration, and critical illness A. Leakage enzymes: alanine aminotransferase or sepsis (ALT), aspartate aminotransferase (AST), sorbitol 4. Uncommon causes of hypocalcemia include dehydrogenase (SDH), and glutamate dehydroge- EDTA contamination, dilution with calcium-free nase (GLDH) IV fluids, intestinal malabsorption, hypovitamin- B. Induced enzymes: alkaline phosphatase (ALP), osis D, pancreatitis, citrated blood transfusions, GGT hypomagnesemia, and tumor lysis syndrome II. Tests for hepatocyte injury D. Hypercalcemia A. Alanine aminotransferase (ALT) 1. Neoplasia is the most common cause of ion- 1. Previously called serum pyruvic transaminase ized hypercalcemia in dogs. Neoplasia is char- (SGPT) acterized by an elevation of both total and ion- 2. Not liver specific; leakage enzyme ized calcium, with parathyroid hormone 3. In dogs and cats, ALT is present mostly in suppressed into the lower part of or below the hepatocytes, but increases can be seen reference range (a parathyroid- independent with muscle injury (especially extensive in- hypercalcemia) jury) a. In dogs, the most common neoplasm caus- 4. Horses and ruminants have little ALT in hepa- ing hypercalcemia is lymphoma. Other tocytes, so elevations of ALT usually indicate neoplasms include anal sac apocrine gland muscle damage adenocarcinoma, thymoma, carcinomas B. Aspartate aminotransferase (AST) (lung, pancreas, mammary gland, skin, na- 1. Previously called serum glutamic oxaloacetic sal cavity, thyroid, adrenal medulla), and transaminase (SGOT) hematologic malignancies (multiple my- 2. Present in hepatocytes and in skeletal and car- eloma, lymphoma, myeloproliferative dis- diac muscle cells ease, leukemia) 3. Not liver specific; leakage enzyme b. In cats, the most common neoplasias are 4. Increased AST can be due to hepatocyte death, lymphoma and squamous cell carcinomas. hepatocyte injury, muscle cell death and mus- Other neoplasms include multiple myeloma, cle cell injury leukemia, osteosarcoma, fibrosarcoma, and 5. Not as specific in the dog and cat as ALT; more bronchogenic carcinoma specific than ALT in horses and ruminants 2. Idiopathic hypercalcemia is the most common C. Sorbitol dehydrogenase (SDH) cause of ionized hypercalcemia in cats. 1. Liver specific; leakage enzyme Idiopathic hypercalcemia is also a parathyroid 2. Increase suggests hepatocyte death or injury independent hypercalcemia 3. Very short half-life, and values return to nor- 3. Renal disease is a common cause of an eleva- mal within a few days tion of serum total calcium but not ionized 4. Not very stable in serum samples; stable for calcium. With renal disease, serum ionized about 48 hours if frozen calcium concentration is typically normal 5. In horses and ruminants, SDH is preferable to to low AST for detecting injury to hepatocytes 4. Vitamin D toxicity from oversupplementation D. Glutamate dehydrogenase (GLDH) with vitamin D, ingestion of plants containing 1. Liver specific; leakage enzyme calcitriol glycosides (Cestrum diurnum), 2. Increase suggests hepatocyte death or injury ingestion of cholecalciferol rodenticides, or 3. More stable than SDH but still not very stable ingestion of antipsoriasis cream (Dovonex). III. Tests for cholestasis Vitamin D toxicity is a parathyroid-independent A. ALP is an induced enzyme hypercalcemia, and an elevation in phosphorus 1. Bone origin (BALP) is typically observed a. Mild increase associated with increased os- 5. Primary hyperparathyroidism causes an teoblast activity elevation of both serum total and ionized cal- b. Will be higher in young growing animals cium with lack of suppression of parathyroid c. May be elevated in association with primary hormone production. Parathyroid hormone or secondary hyperparathyroidism (effects concentration may be still within normal of PTH on bone) limits, or it may be elevated 2. Liver origin (LALP) 6. Other causes of hypercalcemia include a. An increase is associated with cholestasis hypoadrenocorticism, osteolytic processes, b. LALP usually increases before an increase granulomatous disease, grape or raisin toxic- in bilirubin with cholestasis ity, dehydration, vitamin A toxicity, aluminum 3. Corticosteroid-induced (CiALP) toxicity, excessive calcium carbonate supple- a. Induced by corticosteroids and also anti- mentation, intestinal phosphate binders, convulsants thiazide diuretics, acromegaly, or severe b. Chronic disease (including chronic cho- hypothermia lestasis) can induce CiALP  CHAPTER 1 Clinical Pathology: Clinical Chemistry 9 4. In cats, the half-life of ALP is very short (about or indirect. Most bilirubin in horses is un- 6 hours); thus the increase of ALP in choles- conjugated tatic disease is significantly less than in other d. Once in hepatocytes, bilirubin is conju- species gated, and most is secreted into bile. A 5. Increases in horses with cholestasis are not small amount passes through the sinusoidal well documented side back into the blood. Conjugated biliru- 6. In ruminants, increases in ALP are usually due bin is also termed direct bilirubin to cholestasis or osteoblastic activity 2. Abnormal metabolism B. GGT a. Historically, both unconjugated and conju- 1. Considered to be an induced enzyme; however, gated bilirubin have been measured, but acute injury can cause elevations of GGT currently a total bilirubin measurement is 2. Elevated primarily in cholestasis. May also be usually determined induced by glucocorticoids and anticonvulsants b. Bilirubin is increased when there is in- 3. GGT is superior to ALP in horses and rumi- creased hemoglobin production (increased nants for the detection of cholestasis RBC destruction), decreased uptake and 4. GGT is present in very high levels in cattle and conjugation of bilirubin by hepatocytes, and sheep colostrums, resulting in very high levels a decrease in outflow of conjugated biliru- of serum GGT in calves and sheep that have bin (cholestasis, etc.) consumed colostrum c. Bilirubin is not consistently elevated in ru- IV. Tests of liver function minants with liver disease A. Bilirubin B. Bile acids 1. Normal metabolism 1. Fasting and postprandial samples are usually a. The heme portion of hemoglobin is split collected in dogs and cats into iron and protoporphyrin 2. The postprandial bile acid concentration is b. Protoporphyrin is converted to biliverdin, usually greatly exaggerated with portosystemic then to bilirubin shunt c. Bilirubin is released from macrophages, at- 3. Increases in fasting, postprandial, or both sam- tached to albumin or other globulins, trans- ples may occur with portosystemic shunts, ported to the liver, released from albumin cholestasis, cirrhosis, necrosis, hepatitis, he- or globulins, and enters hepatocytes. This patic lipidosis, steroid hepatopathy, and neo- circulating bilirubin is termed unconjugated’ plasia (Figure 1-3) Systemic circulation ↑ Bile acids C B A D Portal venous Biliary circulation system Bile acids Intestines Figure 1-3 The increase in the bile acid level in the circulation is generally caused by one of four disorders: congenital portosystemic shunting (A), hepatic microvascular dysplasia (B), intrahepatic colestatic disease (C), or extrahepatic bile duct obstruction (D). (From Meyer D, Harvey JW. Veterinary Laboratory Medicine: Interpretation and Diagnosis, 3rd ed. St Louis, 2004, Saunders.) 10 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE 4. One sample is collected in horses and rumi- F. Steroid hepatopathy nants. An increase in bile acids suggests he- 1. Most common in dogs patic disease 2. Leakage enzymes are mildly increased C. Ammonia concentration is usually increased in 3. Induced enzymes are markedly increased those with portosystemic shunts or if more than 4. Bilirubin may be mildly increased 60% of liver mass is lost G. Biliary disorders D. Albumin decreases when 60% to 80% of liver func- 1. Induced enzymes are markedly increased tion is lost 2. Leakage enzymes may be mildly increased E. Globulins may be increased, especially in horses as a result of hepatocyte injury from the F. Glucose cholestasis 1. The liver converts glucose to glycogen 3. Bilirubin and bile acids are also typically in- 2. Glucose may be increased if there is decreased creased glucose uptake by the liver H. Chronic liver disease 3. Glucose may be decreased if there is de- 1. Leakage enzymes may be increased, depending creased gluconeogenesis or glycogenolysis on the extent and rate of progression of the G. Urea is synthesized in the liver from ammonia disease 1. BUN decreases with liver failure 2. Induced enzymes are usually mildly to moder- 2. Blood ammonia concentration increases with ately increased liver failure 3. Bilirubin concentration is normal to mildly in- H. Cholesterol creased in those with more advanced disease 1. Can be decreased if there is decreased synthe- I. End-stage liver disease sis of cholesterol with liver failure 1. Occurs when 60% to 80% of liver mass has 2. Can be increased if cholestasis is present, been lost which prevents excretion of cholesterol in bile 2. Leakage enzymes may be normal to mildly in- I. Coagulation factors are commonly decreased in creased because of the overall loss of liver dogs with liver failure mass V. Changes in selected liver diseases 3. Induced enzymes are moderately to markedly A. Portosystemic shunt increased, as are bilirubin and bile acid con- 1. If portosystemic shunts occur because of se- centrations vere cirrhosis, then changes as seen in end- 4. Many have increased ammonia, decreased stage liver disease are expected BUN, decreased albumin, and abnormal coagu- 2. Early portosystemic shunts do not cause much lation tests active hepatocyte damage; thus leakage en- zymes are usually not elevated EVALUATION OF THE PANCREAS a. Induced enzymes are also not elevated be- cause there is little cholestasis I. Pancreatic injury b. Typically occurs in young, growing animals, A. Serum amylase so ALP may be elevated (BALP) 1. Dogs c. Bile acids are markedly elevated a. Highest concentrations in pancreas and d. Microcytic anemia with low iron concentra- small intestinal mucosa tion is typical b. Causes of increased serum amylase include B. Hepatic necrosis pancreatic injury, renal dysfunction, GI dis- 1. If focal, leakage enzymes may be normal or ease, hepatic disease, and neoplasia (lym- mildly elevated phoma, hemangiosarcoma) 2. Diffuse necrosis more often results in eleva- c. Magnitude of the increase may be helpful. tions in both leakage and induced enzymes. If amylase is elevated more than three-fold C. Hypoxia or mild toxic damage greater than the upper reference range 1. This process is diffuse, so leakage enzymes are limit, pancreatic injury is strongly usually mildly to moderately elevated suggested 2. Induced enzymes and bilirubin are not 2. Other species typically elevated a. Amylase is usually normal in cats with pan- 3. Bile acids may be mildly increased creatic injury and may be decreased D. Focal lesions b. Amylase is only slightly elevated in horses 1. Leakage enzymes may be normal to mildly with pancreatic injury and is elevated in increased most horses with proximal enteritis and 2. Induced enzymes are usually normal unless other causes of colic bile flow is significantly impaired B. Serum lipase E. Hepatic lipidosis 1. Dogs 1. Leakage enzymes are increased in most cats a. Causes of increased serum lipase include 2. ALP is also elevated in most, but GGT is ele- pancreatic injury, renal dysfunction, hepatic vated in only a small number disease, GI disease, corticosteroids (dexa- 3. Serum bilirubin is usually elevated, and bile ac- methasone can increase lipase five-fold), and ids are commonly increased neoplasia (lymphoma, hemangiosarcoma)  CHAPTER 1 Clinical Pathology: Clinical Chemistry 11 b. An elevation greater than two-fold is sugges- 3. Dependent on the quantity of starch in the diet tive of pancreatic injury, except if the dog B. Fecal fat has received corticosteroids 1. Direct fecal fat detects undigested fat 2. Cats with pancreatic injury typically have nor- a. Stain feces with Sudan III or IV on slide and mal lipase activity examine C. Peritoneal fluid b. The presence of undigested fecal fat indi- 1. If amylase or lipase activity is higher in perito- cates a deficiency in lipase neal fluid than in serum, pancreatic injury is 2. Indirect fecal fat detects digested fat more likely a. Mix feces, acetic acid, and Sudan III or IV on 2. Consider measuring in cats or horses with sus- a slide; bring to a boil and examine pected pancreatic injury b. The presence of digested fat (in the absence D. Serum trypsin-like immunoreactivity (TLI) of undigested fat) suggests adequate lipase 1. TLI activity is proportional to trypsinogen and production but inadequate absorption of fat trypsin. Trypsinogen is secreted only by the C. Fecal proteolytic activity can be estimated but is pancreas and is converted to trypsin in the rarely performed anymore since the advent of se- small intestine rum TLI determination 2. TLI is increased in pancreatitis and is a more D. Fecal muscle fibers sensitive indicator of early pancreatitis than 1. Stain brown with Lugol stain are amylase or lipase determination 2. Presence suggests inadequate fecal protease 3. TLI is also a sensitive and specific indicator for activity pancreatitis in the cat E. Fat absorption test (plasma turbidity test) II. Exocrine pancreatic insufficiency (see section below 1. After a 12-hour fast, orally administer corn oil, on intestinal absorption) then collect hourly plasma samples for a few hours 2. Turbidity of the samples should occur, indicat- EVALUATION OF DIGESTION ing the absorption of lipid AND INTESTINAL ABSORPTION 3. If turbidity does not occur, then repeat test I. Fecal parasites with corn oil that has been preincubated with A. Refrigerate fecal sample if cannot examine within pancreatic enzymes. If turbidity occurs, then 2 hours absorption occurred and the problem is with B. Direct smears are useful in detecting Strongyloi- digestion of fat des, Coccidia, Giardia, Balantidium, Entamoeba, 4. The sensitivity of this test is poor and Trichomonas spp. F. Serum TLI C. Wet mounts are useful to detect Giardia, Balantid- 1. Available for dogs and cats ium, and Entamoeba spp. 2. TLI is decreased in dogs and cats with EPI. It is D. Fecal flotation normal in other small intestinal diseases 1. Best method for detecting parasitic ova and G. D-Xylose absorption test oocysts 1. Measure of intestinal absorption in dogs and 2. Different fecal flotation solutions can be used, horses including a sugar solution, sodium chloride, 2. Xylose absorption is falsely decreased in those magnesium sulfate, zinc sulfate, or sodium ni- with delayed gastric emptying, bacterial over- trate solutions growth, and in some with exocrine pancreatic E. Baermann technique is most useful for detection insufficiency (EPI) of larvae in feces. H. Vitamin B12 and folate assays II. Fecal occult blood 1. Serum folate is decreased if there is malab- A. Performed in animals with chronic diarrhea or sorption in the proximal small intestine loose stools, microcytic anemia, a history of 2. Serum vitamin B12 is decreased if the malab- GI tumors, or in those treated with NSAIDs sorption is primarily in the distal small B. Positive test result suggests upper GI tract inflam- intestine mation, ulceration, or neoplasia 3. In cats with EPI, both serum vitamin B12 and fo- C. False positives may occur when consuming meats late levels are usually decreased. In dogs with or some vegetables. It is best to restrict the diet EPI, serum vitamin B12 is usually decreased, (rice and cottage cheese) for a few days before and folate is usually normal to increased the test 4. In small intestinal bacterial overgrowth, III. Fecal cytology vitamin B12 is decreased and folate is increased A. Look for types of bacteria present B. Evaluate for presence of inflammatory cells EVALUATION OF SERUM IV. Digestion and absorption tests AND PLASMA PROTEINS A. Fecal starch 1. Stain feces with Lugol solution I. Plasma versus serum 2. The presence of undigested starch suggests a A. Plasma contains albumin, and all globulins, which deficiency in starch-digesting enzymes or in- include antibodies, clotting factors, enzymes, and creased intestinal motility proteins 12 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE B. Serum contains no fibrinogen and only contains exocrine pancreatic insufficiency, and glomeru- albumin and remaining globulins lar disease (loss of albumin) II. Total protein concentration 3. Hypoglobulinemia with normal or increased al- A. Can be measured with a refractometer. Excess bumin. Causes include failure of passive trans- lipid, hemoglobin, bilirubin, glucose, urea, fer, and immune deficiencies (inherited or ac- sodium, or chloride can falsely increase quired) total protein concentration as measured by B. Increased protein concentrations refractometry 1. Hyperalbuminemia occurs only in dehydration B. Spectrophotometric measurement is more accu- 2. Hyperalbuminemia with hyperglobulinemia oc- rate curs in dehydration III. Albumin concentration 3. Hyperglobulinemia A. Measured spectrophotometrically a. Increased -globulin concentration occurs B. At very low concentrations, albumin may be over- most commonly in acute inflammation estimated b. Increased -globulin concentration occurs IV. Globulin concentrations in acute inflammation, nephrotic syndrome, A. Fractions liver disease, and immune responses 1. The -fraction includes thyroxine-binding glob- c. Increased -globulin concentration (gam- ulin, transcortin, some lipoproteins (LPs), mopathies) ceruloplasmin, haptoglobin, antithrombin III, (1) Polyclonal gammopathies are present and 2-macroglobulin with chronic antigenic stimulation, 2. The fraction includes some LPs, transferrin, immune-mediated disease, liver ferritin, C-reactive protein, complement C3 and disease, lymphoma, and lymphocytic C4, plasminogen, and fibrinogen (in plasma leukemia only) (2) Monoclonal gammopathies are present 3. The -fraction includes the immunoglobulins with multiple myeloma, extramedullary B. Measurement plasmacytoma, lymphoma, lymphocytic 1. The globulin concentration reported on a leukemia, chronic pyoderma, plasma- chemistry profile is calculated by subtracting cytic enterocolitis, canine ehrlichiosis, serum albumin from total protein concentra- visceral leishmaniasis (dog), lympho- tion plasmacytic stomatitis (cats), and idio- 2. Accurate measurement is determined by se- pathic monoclonal gammopathy rum protein electrophoresis C. Hyperfibrinogenemia occurs in dehydration, in- 3. Estimation of immunoglobulins flammation, renal disease, disseminated neopla- a. Refractometry sia, and during terminal pregnancy in cattle (1) Not reliable in foals (2) Cutoff value for plasma protein concen- DETECTION OF MUSCLE tration that indicates adequate passive INJURY (Figure 1-4) transfer (3) Dehydration can falsely elevate plasma I. Creatine kinase (CK) protein concentration A. Also referred to as creatine phosphokinase b. Sodium sulfite precipitation test B. Located in the cytoplasm of skeletal muscle, car- (1) Determines three general ranges for im- diac muscle, and smooth muscle. munoglobulin quantities in calves 1. Considered to be a muscle-specific enzyme (2) Not reliable for foals even though it is also found in the brain and c. Zinc sulfate turbidity test nerves (1) Can be used in both calves and foals 2. Increased CK activity has not been observed in (2) Better tests available injury to the central nervous system d. Glutaraldehyde coagulation test C. May be falsely elevated with hemolysis, hyperbili- (1) Used in calves and foals rubinemia, and muscle fluid contamination of a (2) Use serum blood sample during difficult venipuncture V. Fibrinogen concentration D. Increased CK is caused by skeletal muscle injury, A. Most common method is by heat precipitation cardiac muscle injury, or with muscle catabolism B. Can also measure in citrated blood (expensive, (especially in cats) not routinely used) E. The magnitude of the increase in CK does not cor- VI. Abnormal concentrations relate to the severity of the injury A. Decreased protein concentrations F. Serum CK rapidly increases after injury and rap- 1. Hypoalbuminemia with hypoglobulinemia. idly decreases when the injury ceases (normal Causes include blood loss, protein-losing enter- within 24 to 48 hours) opathy, severe exudative skin disease, severe II. Aspartate aminotransferase (AST) burns, and effusive disease A. Previously known as serum glutamic oxaloacetic 2. Hypoalbuminemia with normal or increased transaminase (SGOT) globulins. Causes include starvation, liver dis- B. Present in cytoplasm and organelles of hepato- ease, GI parasites, intestinal malabsorption, cytes, skeletal muscle, and cardiac muscle  CHAPTER 1 Clinical Pathology: Clinical Chemistry 13 Dog and Cat 2. If urine is positive for hemoglobin on a dipstick ↑ ALT greater than ↑ AST; no ↑ CK → liver injury and serum is red, then hemoglobinuria is prob- ↑ AST greater than ↑ ALT; ↑ CK → skeletal muscle injury ably present Horse and Ruminant ↑ AST; no ↑ CK → liver injury D. Addition of ammonium sulfate to produce an 80% ↑ AST and ↑ CK → skeletal muscle injury or concomitant concentration in urine will cause hemoglobin to skeletal muscle injury and liver injury precipitate. The resultant supernatant will test negative for hemoglobin because the hemoglobin has precipitated. If myoglobinuria is present, the supernatant will continue to test positive for he- CK moglobin with dipstrip EVALUATION OF LIPIDS I. Lipid metabolism AST A. Triglyceride 1. Triglycerides are composed of a glycerol ALT backbone to which three fatty acids (of varying length and characteristics) are Reference attached range 2. When ingested, triglycerides are broken down 2 4 6 8 10 to monoglycerides and free fatty acids for Approximate time (days) following severe absorption by the intestinal mucosal cells. skeletal muscle injury with resolution Absorption requires bile acids and micelle for- mation. Once in the intestinal mucosal cells, Figure 1-4 The approximated magnitude and duration of increase in CK, triglycerides are formed from the absorbed AST, and ALT levels in the circulation after severe injury to the skeletal monoglycerides and free fatty acids muscle in all domestic species are illustrated. In dogs and cats, the relative magnitude of ALT, AST and CK levels help differentiate predominantly liver 3. Hypertriglyceridemia normally occurs in or skeletal muscle injury. In horses and ruminants, a rise in the AST level with the postprandial state. Increases in serum tri- or without a notable change in the CK level is compatible with liver injury. glyceride concentrations are also noted in The AST and CK levels are increased in horses and ruminants with skeletal some primary lipid disorders and in second- muscle injury alone or with concomitant liver injury. The latter is indicated ary lipid disorders such as pancreatitis, by an increase in the sorbitol dehydrogenase or GLD levels. ALT, Alanine hypothyroidism, diabetes mellitus, nephrotic aminotransferase; AST, aspartate aminotransferase; CK, creatine kinase. syndrome, hyperadrenocorticism, and (From Meyer D, Harvey JW. Veterinary Laboratory Medicine: Interpretation cholestasis and Diagnosis, 3rd ed. St Louis, 2004, Saunders.) 4. Hypercholesterolemia may falsely decrease se- rum triglyceride measurement C. Increases more slowly than CK with injury and B. Cholesterol persists in the serum longer 1. Present only in animal tissues D. Useful in combination with CK to determine when 2. Can be produced in almost any tissue in the muscle damage has occurred. body, but the major sites of cholesterol synthe- III. ALT sis are the liver and small intestine A. Previously known as SGPT 3. Absorbed by the small intestine; absorption re- B. Some elevation may be seen in dogs and cats quires bile acids and micelle formation with muscle injury and no hepatic damage 4. Hypercholesterolemia is noted in primary C. Small amounts of activity in skeletal and cardiac lipid disorders and also in association with muscle but can contribute to elevated ALT be- hypothyroidism, diabetes mellitus, nephrotic cause the muscle mass is large syndrome, pancreatitis, hyperadrenocorticism, IV. Lactate dehydrogenase (LDH) and cholestasis. Cholesterol is elevated A. Located in the cytoplasm in most cells and thus normally in the postprandial period is nonspecific for muscle 5. Hypertriglyceridemia and hyperbilirubinemia B. Five isoenzymes exist, and analysis of isoenzymes may falsely lower the serum cholesterol con- may be more beneficial centration V. Myoglobin C. LPs are conglomerates of various apoproteins, A. Released from dead muscle cells into the blood cholesterol, triglyceride, and phospholipids (myoglobinemia) and excreted into the urine 1. Metabolism (myoglobinuria) a. LPs are classified according to their density B. Myoglobinuria results in a dark brown to red- as chylomicrons: very-low-density brown coloration of the urine lipoproteins (VLDLs), intermediate-density C. Myoglobin is rapidly excreted by the kidneys and lipoproteins (IDLs), and high-density lipo- thus serum remains colorless to yellow proteins (HDLs) 1. If urine is positive for hemoglobin on a dipstick b. Chylomicrons are formed primarily by and serum is colorless to yellow, then myoglo- the small intestine and are composed of binuria is probably present high quantities of triglyceride, with low 14 SECTION I GENERAL DISCIPLINES IN VETERINARY MEDICINE amounts of protein. They are the least C. Lipoprotein electrophoresis dense of the LPs and will ‘float’ in a 1. Separates LPs based on their charge and mo- serum sample that contains excess bility on agarose gel chylomicrons (hyperchylomicronemia). 2. If submitting a serum sample for lipoprotein elec- Chylomicrons are normally cleared trophoresis, make sure the laboratory has expe- rapidly by the liver after a meal. Lipoprotein rience with animal samples and interpretation lipase is required for proper chylomicron of LP patterns; otherwise, erroneous interpreta- metabolism tions will occur c. VLDLs are formed by the liver; they are 3. Most useful for monitoring the effectiveness of smaller than chylomicrons and have a treatment of lipid abnormalities higher content of protein and lower triglyc- III. Hyperlipidemia eride level. They are heavier than chylomi- A. Postprandial hyperlipidemia is the most common crons. Once in the circulation, exchanges of cause of hyperlipidemia proteins and constituents occur between B. Equine hyperlipidemia LPs, and IDLs are formed. IDL is rapidly 1. Occurs primarily in miniature horses, ponies, converted to LDL. Lipoprotein and donkeys lipase activity is required 2. Caused by starvation and chronic illness, cre- d. LDLs are heavier and smaller than VLDLs ating a negative energy balance with mobiliza- and have a higher protein content and a tion of triglycerides lower triglyceride content. They are capable C. Primary hyperlipidemia of delivering cholesterol to many tissues via 1. Idiopathic hypercholesterolemia the LDL receptor a. First observed in briards but has been seen e. HDLs are formed mostly by the liver and are in other breeds as well integral in the return of cholesterol from b. Marked increase in serum cholesterol tissues to the liver (reverse cholesterol with generally normal triglyceride transport). HDLs are the heaviest and small- concentrations est LPs and have the highest protein and c. In dogs, an increase in HDL1 is noted lowest triglyceride content 2. Idiopathic or primary hyperlipoproteinemia 2. Species differences a. Most likely, many different syndromes can a. Most animal species are HDL mammals, cause idiopathic hyperlipoproteinemia. meaning that most of the cholesterol is car- b. One variant in dogs has been shown to ried by HDLs. HDL mammals include dogs, be due to a decrease in lipoprotein lipase cats, horses, ruminants, rodents, and most activity other species. Pigs, rabbits, guinea pigs, c. A primary hyperlipoproteinemia in lambs is hamsters, camels, rhinoceros, most mon- the result of decreased lipoprotein lipase keys, and humans are LDL mammals, activity meaning that most cholesterol is carried d. Associated with miniature schnauzers by LDLs but also occurs in many other breeds b. In LDL mammals, cholesterol ester transfer (especially Shetland sheepdogs) protein (CETP) transfers most of the choles- 3. Hyperchylomicronemia of cats terol from HDLs to LDLs; thus LDL is the a. Due to a decrease in lipoprotein lipase major carrier of cholesterol activity c. HDL mammals have a low level of CETP, b. Clinically well characterized; affected cats and cholesterol is not transferred to have xanthomas, nerve dysfunction, lipemia LDLs; thus HDL transports most of the retinalis, and anemia. Extreme elevations of cholesterol triglyceride and cholesterol have been d. Lipoprotein characteristics of most animal noted species are very different from human LPs D. Secondary hyperlipidemia occurs as a result of II. Diagnostic approach to hyperlipidemia other disease processes A. Serum turbidity 1. Causes include hypothyroidism, diabetes 1. Hypertriglyceridemia causes serum turbidity mellitus, hyperadrenocorticism, pancreatitis, 2. The opacity of the serum correlates to serum nephrotic syndrome, cholestasis, and the triglyceride content. Serum with the appear- ingestion of diets very high in fat (sled-dog ance of whole milk can have triglyceride con- diets) centrations as high as 2500 to 4000 mg/dL 2. Most of the lipid abnormalities resolve B. Refrigeration test with treatment of the underlying 1. Place the serum sample in the refrigerator condition overnight IV. Hypolipidemia 2. Chylomicrons will float, forming a “cream A. May occur with liver failure layer” B. Decreases in triglyceride may be seen in associa- 3. If the serum below the chylomicron layer is tion with maldigestion and malabsorption syn- still turbid, then other LPs (most likely VLDLs dromes, lymphangiectasia, and portosystemic or LDLs) are present in excess shunts  CHAPTER 1 Clinical Pathology: Clinical Chemistry 15 K. Drugs (glucocorticoids, progesterone, EVALUATION OF GLUCOSE METABOLISM xylazine, ketamine, morphine, phenothiazine, ad- I. Hypoglycemia. Causes include the following: renocorticotrophic hormone, glucose-containing A. Starvation or malabsorption; occurs only after fluids) long-term starvation L. Milk fever (cattle) B. Increased insulin production (insulinoma); M. Neurologic diseases (cattle); from increased glu- reported in dogs, cats, and ferrets cocorticoid and epinephrine concentrations C. Insulin overdose N. Proximal duodenal obstruction (cattle); causes D. Hypoadrenocorticism extreme hyperglycemia due to decreased periph- 1. Occasionally, a mild hypoglycemia is seen eral glucose use 2. A result of decreased gluconeogenesis and in- O. Colic (horses); horses with very high glucose creased insulin-mediated uptake of glucose by concentrations have a poor prognosis muscle P. Hyperthyroidism; occurs in a small percentage of E. Growth hormone deficiency cats and may be due to stress or other causes of 1. Hypoglycemia is uncommon hyperglycemia 2. Occurs if current hypoadrenocorticism is Q. Moribund condition (ruminants) present III. Blood glucose analysis F. Liver failure A. Fast dogs and cats for 12 hours to avoid post- 1. From decreased hepatic gluconeogenesis and prandial hyperglycemia. Do not fast if hypoglyce- glycogenolysis mia is suspected 2. Occurs after greater than 70% hepatic function B. Horses and cattle are typically not fasted has been lost C. Separate serum or plasma from cells within G. Portosystemic shunt (if hepatic dysfunction 30 minutes of collection. Glycolysis of RBCs will is severe) result in a 10% glucose decrease per hour if not H. Extreme exertion (hunting dogs and horses) separated I. Juvenile hypoglycemia in toy and miniature dogs D. Minimize stress and excitement in cats before col- J. Glycogen storage diseases lecting sample K. Sepsis IV. Urine glucose analysis 1. More common with advanced sepsis A. Glucose appears in the urine when the renal 2. Possibly because of impaired gluconeogenesis threshold for glucose has been exceeded and glycogenolysis and increased utilization of 1. Dogs and cats; approximately 180 to 200 mg/dL glucose by tissues 2. Horses; approximately 180 mg/dL L. Ketosis (cattle); negative energy balance leads to 3. Cattle; approximately 100 mg/dL hypoglycemia with an increase in ketone bodies B. The renal threshold may be decreased with production proximal tubular abnormalities, Fanconi syn- M. Pregnancy tox