Chapter 11: Nonprotein Nitrogen and Renal Function PDF

CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 261 INTRODUCTION The kidneys play a vital role in maintaining levels of many sub-stances in the human body, retaining critical...

CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 261 INTRODUCTION The kidneys play a vital role in maintaining levels of many sub-stances in the human body, retaining critical components and elimi-nating what is not essential. The most crucial roles are removal Kidney of waste, toxic, and surplus products from the body; homeostasis of the bodys water; regulation of acidbase levels and electro-lytes; and hormonal regulation. Renal function testsfor exam-ple, blood urea nitrogen (BUN), creatinine, and electrolytesare included in chemistry screening profiles to screen for renal disease, water balance, and acidbase disorders. Nonprotein nitrogen was the original test of renal function. NPN comprises products from the catabolism of proteins and Ureter nucleic acids that contain nitrogen but are not part of a protein molecule. Earlier NPN testing required a protein-free filtrate (PFF): precipitating protein from the serum and then centrifuging and using the PFF supernatant for testing. This procedure was time-consuming and technically difficult. This chapter will concen-trate on procedures developed to measure individual NPN com-pounds, procedures that are much simpler and can be automated. Bladder RENAL ANATOMY The urinary system consists of two kidneys, two ureters, a bladder, Miucci/Shutterstoc Urethra and urethra, asillustrated in Figure 11-1. The kidneys are located in the posterior abdominal wall and are approximately 12 cm long, FIGURE 11-1 Urinary system. 6 cm wide, and 2.5 cm in depth; each weighs approximately 140 g. The kidneys are divided into two distinct areas: the outer layer, or cortex, and the inner layer, or medulla. Each kidney contains approxi-mately 1 million to 1.5 million nephrons, the functional unit of the kidney (see Figures 11-2 and 11-3 ). Calyces Renal capsule Cortex Medulla Renal artery Renal vein Renal pelvis Ureter Ducu59us/Shutterstock FIGURE 11-2 Kidney.  262 CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN Blamb/Shutterstoc FIGURE 11-3 Nephron. Renal blood flow is vital to renal function. Blood is supplied to RENAL PHYSIOLOGY the kidney by the renal artery from the abdominal aorta and enters The three major renal functions are glomerular filtration, tubu-lar the nephron through the afferentarteriole.It flows through the glo-merulus reabsorption, and tubular secretion. Glomerular filtration into the efferent arteriole. The glomerulus consists of a coil occurs in the glomerulus, which is the first part of the nephron. of approximately 40 capillary loops referred to as the capillary tuft Filtration is enhanced by several factors. First, the pressure in the located within the Bowmans capsule, the initial section of the neph-ron. glomerular capillaries is high because of the difference in size The blood is filtered in the glomerulus, and the filtrate flows between the afferent and efferent arterioles, described earlier. Sec-ond, through the proximal convoluted tubule (PCT), the descending the semipermeable glomerular basement membrane allows loop of Henle, the ascendingloop of Henle, the distal convoluted tubule (DCT), the collecting duct, the renal calyces,the ureters, the bladder, and the urethra, in that order. The collecting duct from each nephron combines with other collecting ducts to form the renal calyces, BOX 11-1 Urinary Filtrate Flow where urine collects before passing into the ureters, bladder, and urethra. Box 11-1 outlines the flow of urinary filtrate. 1. Bowmans capsule Renal blood flows from the afferent arteriole to the efferent 2. Proximal convoluted tubule (PCT) arteriole, and the smaller diameter of the efferent arteriole results 3. Descending loop of Henle in a hydrostatic pressure differential that is important for glo-merular 4. Ascending loop of Henle filtration, which will be discussed later. From the efferent 5. Distal convoluted tubule (DCT) arteriole, the blood enters the peritubular capillaries and flows slowly 6. Collecting duct through the cortex and medulla, where the capillaries divide into the vasa recta. The peritubular capillaries surround the proximal and 7. Renal calyces distal convoluted tubules and are responsible for the immediate 8. Ureter reabsorption of essential substances from the fluid in the PCT. 9. Bladder The vasa recta lead to the renal vein, where blood is returned to the 10. Urethra body. Renal blood flow is outlined in Box 11-2.  CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 263 they pass through the peritubular capillaries, they develop a strong BOX 11-2 Renal Blood Flow bond for the tubular cells and dissociate from their carrier pro-tein, resulting in their transportation into the filtrate by the tubular 1. Renal artery cells. The major site for removal of these nonfiltered substances 2. Afferent arteriole (H+, NH3, K+, and weak acids and bases) is the PCT. 3. Glomerulus 4. Efferent arteriole ANALYTES ASSOCIATED 5. Peritubular capillaries WITHRENAL FUNCTION 6. Vasa recta 7. Renal vein Nonprotein Nitrogen Nonprotein nitrogen (NPN) comprises the products of the catabolism of proteins and nucleic acids that contain nitro-gen but are not part of protein molecules. The kidneys remove low-molecular-weight molecules less than 66,000 daltons to pass excess NPN from the body; therefore, one of the major reasons through into the filtrate. Albumin and other low-molecular-weight for measuring NPN is to evaluate renal function. Approximately proteins, glucose, amino acids, urea, and creatinine are freely fil-tered 15 NPN compounds are found in plasma. The clinically significant and proceed to the proximal convoluted tubules. Third, the NPN compounds discussed in this chapter are urea nitrogen, uric basement membrane is negatively charged, so large, negatively acid, and creatinine. Ammonia will becoveredin Chapter19 Liver charged molecules (e.g., proteins) are repelled. Function. The most common NPN compounds and their relative Tubular reabsorption occurs through active transport and percentage concentrations are listed in Table 11-1 ?. passivetransport. Every minute, 1200 mL of ultrafiltrate is filtered through the 2 million nephrons. Obviously, the body cannot lose Ureaor Blood Urea Nitrogen 1200 mL of water containing essential nutrients every minute. The Urea or blood urea nitrogen (BUN) is the major nitrogen-containing PCTs reabsorb approximately 80% of the fluid and electrolytes fil-tered metabolic product of protein catabolism in humans. It by the glomerulus, including 70% of the sodium and chloride is formed from exogenous protein (protein in the diet) or endog-enous and most of the potassium, phosphate, and sulfate. protein from the breakdown of cells in the body. Approxi-mately In active transport, the substance to be reabsorbed must be 75% of the NPN in the body is eventually excreted. Alarge combined with a carrier protein contained in the membranes of percentage of protein is converted to urea during the breakdown the renal tubular cells. It is transported against a concentration of protein. Figure 11-4 shows the chemical structure of urea. gradient and requires the expenditure of energy from adenosine The synthesis of urea is carried out exclusively by the hepatic triphosphate (ATP). Sodium is moved through active transport in enzymes of the Krebsand Henselheit ureacycle.In the intestine, urea the proximal convoluted tubule, the ascending loop of Henle, and is metabolized byintestinal bacteria to form ammonia and carbon the collecting tubules. When the concentration of the substance dioxide. The ammonia is reabsorbed through the portal system (a (e.g., glucose) exceeds the capacity of the active transport system, venous system comprising the hepatic portal vein and its tributar-ies) the substance is excreted or spilled into the urine. The renal and carried to the liver, whereit is reconverted to urea, which threshold is the concentration above which the substance cannot is less toxic. More than 90% of the urea is excreted by the kidneys, be totally reabsorbed and is excreted in the urine. For example, the and the remainder is lost through the gastrointestinal tract and skin. renal threshold of glucose is 160 to 180 mg/dL. If an individuals There is a direct relationship between urea and the serum glucose exceeds the renal thresholdfor example, with dia-betesthe glomerular filtration rate (GFR). Urea is freely filtered in urine glucose will be positive. patients with normally functioning kidneys. In a patient with a Passive transport requires no energy and is characterized by normal to increased GFR, approximately 40% of the BUN is the movement of a substance from an area of high concentra-tion reabsorbed and 60% excreted; therefore, a well-hydrated patient to one of lower concentration. The mode of transport also excretes more BUN, resulting in a lower serum BUN. In a dehy-drated depends on the location in the nephron; for example, chloride is patient, 70% of the BUN is reabsorbed and 30% excreted; absorbed actively in the ascending loop of Henle and passively in the proximal convoluted tubule. Water and urea are always reab-sorbed through passive transport. ? TABLE 11-1 Nonprotein Nitrogen Compounds and Tubular secretion is the opposite of tubular reabsorption Relative Concentration (%) and involves the passage of substances from the peritubular capil-laries into the tubular filtrate. Tubular secretion serves two major Blood urea nitrogen (BUN) 45% functions: elimination of waste products not filtered by the glom-erulus Amino acids 20% and regulation of acidbase balance in the body through Uric acid 20% secretion of 90% of the hydrogen ions excreted by the kidney. Creatinine 5% Many foreign substances (e.g., medications) cannot be filtered by Creatine 12% the glomerulus because they are bound to proteins that makethem Ammonia 0.2 too large to pass through the glomerular pores. However, when 264 CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN O BOX 11-3 Prerenal, Renal, NH2 C NH2 and Postrenal Azotemia Urea Prerenal azotemia FIGURE 11-4 The chemical structure of urea. Decreased renal blood flow Congestive heart failure asa result, the patients serum BUN is increased and urine BUN is Dehydration decreased.BUN is dependent onthree variables:ureaconcentra-tion, Shock (from blood loss) glomerular filtration rate, and level of hydration. Advanced cirrhosis Septic states BUN Clinical Significance Increased protein catabolism Manydifferent renal diseasesare associated with an elevated BUN; Muscle wasting (starvation) however,there are manynonrenalfactors that alsoresult in high Gastrointestinal hemorrhage BUNs. Uremia is anincrease in urea and BUN. In azotemia, there Stress is an increase in blood urea and other NPN compounds. Azote-mia is classifiedin three categories:prerenal,renal, and postrenal. Steroids Prerenal azotemia occurs before the kidney, usually because of Uncontrolled diabetes mellitus a decreased renal blood flow. Examples of conditions associated High fever with prerenal azotemia are dehydration (most common cause), Renal Azotemia congestive heartfailure, shock, and advanced cirrhosis. Anincrease in protein catabolism is also associated with prerenal azotemia; Uremia e.g.,gastrointestinal hemorrhage,steroids, highfever, and muscle Acute kidney injury wasting (starvation).1 Glomerulonephritis Renal azotemia is associated with kidney disease;for exam-ple, Nephrotic syndrome glomerulonephritis, nephrotic syndrome,acute kidneyinjury, Acute renal failure and acute renal failure. In renal disease,the GFRis decreased and less BUN is excreted in the urine.1 Postrenal Azotemia Postrenal azotemia occurs after the urine hasleft the kid-ney Tumors of the bladder or prostate gland and results from an obstruction of urine flow through the Prostatic hypertrophy kidneys, bladder, or urethra. Tumors of the bladder or prostate Gynecologic tumors gland, prostatic hypertrophy, nephrolithiasis(kidney stones), and Nephrolithiasis severe infections are examples of postrenal azotemia.2 Box 11-3 Severe infections hasa morecompletelisting of prerenal,renal, and postrenalcauses of azotemia. Decreased BUN is found in five conditions: (1) decreased bleeding; the higher the ratio, the more likely the bleeding is from proteinintake, (2) severeliver disease,(3) overhydration,(4) dur-ing an upper GI source.4 the third trimester of pregnancy (because of increased plasma Decreased ratios are much less common but can be seen in volume), and (5) syndrome of inappropriate antidiuretic hormone secretion (SIADH).3 In liver failure, the Krebs and Henselheit cycle renal dialysis patients because BUN is more dialyzable than CR. Other conditions associated with a low ratio are acute tubular is no longer effective, so BUN is not synthesized. During preg-nancy, necrosis, low-protein diets, starvation, severe diarrhea, vomit-ing, the fetus is using maternal amino acids, so less protein is availableto be catabolizedto BUN. syndrome of inappropriate antidiuretic hormone secretion (SIADH), rhabdomyolysis, and cirrhosis.3 The BUN:creatinine (BUN:CR) ratio can be used to dis-tinguish between the three majortypes of azotemia. The normal ratio is between 10:1and 20:1.In renal disease,becausethe BUN BUN Methodologies and CR are both elevated proportionally, the ratio will fall within BUN methodologies can be categorized as enzymatic urease reac-tions the normalrange. A highratio of 720:1to 30:1 witha high BUN and the diacetyl or Fearon reaction. The mostcommon BUN and a normal or onlyslightly elevated CRis associated with prer-enal methodologies utilize urease in the initial reaction. azotemia.1 Highratios with an elevated CRsuggest postrenal obstruction (azotemia) or prerenal azotemia in addition to renal disease.For example,a patient whohasa BUN of 45 mg/dLand Urea + 2H2O + + CO3-2 Urease 2NH4 (Eq. 11-1) a CR of 1.3 mg/dL would have a ratio of 35 (45/1.3) (a moder-ately elevated BUN and only slightly elevated CR), which would beindicative of prerenalazotemiafor example,from congestive Thereaction then proceedsto Bertholots reaction, Nesslers heartfailure. Aratio 730is suggestive of upper gastrointestinal reaction, or glutamate dehydrogenase  CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 265 In Berthelots reaction, the ammoniumion is reacted with phenol A BUN of 68910 mg/dL probably resultsfrom overhydra-tion. and hypochlorite in an alkaline medium to form indophenol blue, Other causes include a diet very low in protein, malnutrition, the chromagen that is measured. Sodium nitroprusside is added to orliver disease.3 A BUN of 50150 mg/dL is clearly abnormal and catalyze the reaction. beyond variation from urine flow or nitrogen load and indicates impairment of the glomerular filtration rate. A BUN of 150250 mg/dL is conclusive evidence of severe renal impairment. NaOH NH4+ + 5NaOCl + Phenol Nanitroprusside Indophenol blue + It is useful to know how to convert from urea to urea nitrogen (BUN) and vice versa (to understand journal articles in European 5NaCl + 5H2O (Eq. 11-2) publications, for example). The conversion factor is calculated using the molecular weightof BUN (60) and nitrogen(2 * 14 = 28). In Nesslersreaction, the addition of a double iodide compound To convert from urea to urea nitrogen, divide the molecular weight (2HgI2 + 2KI) resultsin the formation of a yellow to of nitrogen orange-brown by the molecular weight of urea (28/60), for a factor compound with NH4+ of 0.467. Therefore,a urea of 56 mg/dL * 0.467is equivalentto. a urea nitrogen (BUN) of 26 mg/dL (rounding off to the nearest whole number). 2HgI2 + 2KI + NH4+ S NH2Hg2I3 + 4KI + NH4I To convert urea nitrogen to urea, the factor is (60/28), or 2.14. (Eq. 11-3) CHECKPOINT! 11-1 The glutamate dehydrogenase procedure is the most commonly 1. Ajournal article reports a urea nitrogen of 10 mg/dL. used. The disappearance of NADH is measured as a decrease in What would be the equivalent urea concentration? absorbance as NADH is oxidized to NAD+. 2. Classify the following as prerenal, renal, or postrenal azotemia. NH4+ + 2@oxoglutarate+ NADH Glutamate dehydrogenase a. Dehydration NAD+ + Glutamate + H2O (Eq. 11-4) b. Glomerulonephritis c. Congestive heart failure d. Nephrolithiasis The diacetyl or Fearon reaction is a colorimetric reaction based e. Shock on the condensation of diacetyl with urea to form the chromogen diazine. 3. A BUN:CR ratio of 15 with a moderately elevated BUN and creatinine is associated withprerenal, renal, or postrenal azotemia? H+ Urea + Diacetyl + H2O Strong acidDiazine+ 2H2O (Eq. 11-5) CREATININE Creatine is synthesized in the liver from three amino acids: argi-nine, The reaction of diacetyl and urea results in a diazine derivative glycine, and methionine. Creatinine (CR) is a waste product that absorbs strongly at 540 nm. Thiosemicarbazide and Fe(III) derived from creatine and creatine phosphate. Creatine is produced ions are added to stabilize and enhance the reaction. when creatine phosphate (phosphocreatine) loses a phosphoric Aconductimetric method is based on the increase in conductivity, acid molecule during the process of musclecontraction, and creati-nine where the sample is injected into urease reagent in a reaction cup is an anhydride formed when creatine loses a water molecule: containing an electrode that measures conductivity. The increase is directly proportional to the ammonium ions produced in the Creatine kinase urease reaction. Phosphocreatine + ADP Creatine + ATP (Eq. 11-6) BUN Reference Range Blood urea nitrogen is the term still used, although laboratories no longer test whole blood, and in the United States it is reported out Creatine S Creatinine + H2O (Eq. 11-7) as urea nitrogen, not urea. The reference range is 718 mg/dL. The reference range for BUN on a 24-hour urine is 12 to 20 grams/24h. BUN is stable at room temperature for 24 hours, several days at The constancy of endogenous creatinine production is pro-portional refrigerator temperature (4C), and 2 to 3 months frozen. A high-protein to the muscle mass of the individual, and creatinine is diet increases BUN, but a single meal would have negligible released into the body fluids at a constant rate. The constant plasma effect. Although BUN is slightly higher in males,the gender differ-ence levels over a 24-hour (24h) period make creatinine a good endog-enous is not clinically significant. substance to use as an indicator of glomerular filtration  266 CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN Creatinine is readily filtered by the glomeruli and does not undergo Creatininase may become the method of choice with less any significant tubular reabsorption. interference, more accuracy, and greater precision. Creatinine levels are affected by three main variables: relative A second enzymatic procedure also uses creatinase, followed muscle mass, creatine turnover, and renal function. by creatine kinase, pyruvate kinase (PK), and lactate dehydrogenase (LD) (creatinase S CKSPKS LD). Creatinine Clinical Significance Creatinine is primarily an index of renal function and measures the Creatinine + H2o Creatinase Creatine (Eq. 11-12) GFR. Increased serum creatinine is present when the formation or excretion of urine is impaired because of prerenal, renal, or postrenal causes. Creatinine values are usually not above the upper Creatine kinase reference limit until one-half to two-thirds of renal function is lost. Creatine + ATP Creatine phosphate + AdP Low values are not clinically significant. Serum CR levels and (Eq. 11-13) urinary CR excretion are functions of muscle massin normal indi-viduals and show little response to dietary changes. High levels of CRin urine are unique to the kidney. CRlevels in urine can be Pyruvate kinase used to detect falsely low CRif a urine has been diluted (e.g., drug AdP + Phosphoenolpyruvate ATP + Pyruvate testing). CR and BUN levels can be utilized to identify a fluid as (Eq. 11-14) urine because urine has the highest concentrations of these two substances. Most urine samples have a creatinine level of 20250 mg/dL;lessthan 2% ofspecimenshavea creatinine 620 mg/dL, Pyruvate + NAdH lactate + NAd+ lactate dehydrogenase which are considered dilute. (Eq. 11-15) Creatinine Methodologies The Jaffe reaction is a reaction between creatinine and picric acid This procedure requires a larger sample size and is not as in an alkaline medium, yielding a red-orange compound of cre-atinine popular as the peroxidase reaction. The National Kidney Disease and a picrate ion. Concentration or alkalinity of reagent is Education Program has recommended the standardization of critical in the Jaffe reaction. Interfering substances include protein, creatinine calibration to be traceable to an isotope dilution mass glucose, uric acid, ascorbic acid, acetone, ketoacids, and medica-tions spectrometry (IDMS) reference method procedure.5 (cephalosporins and other antibiotics). A kinetic method was developed to reduce the effect of Creatinine Reference Range interfering substances. The absorbance at 520 nm is measured The creatinine reference range for men is 0.91.2 mg/dL, and between 20 and 80 seconds. Certain interfering substancesfor slightly lower for women (0.61.1 mg/dL) because of lower example, acetoacetatereact faster and others such as protein are muscle mass. Creatinine levels decrease with age beginning in slower; therefore, creatinine is the main reactant between 20 and the fifth decade. Intraindividual variation is small, and creatinine 80 seconds. is not affected by diet unless it ultimately affects muscle mass. An enzymatic methodology using creatininase (creatinine Intense exercise can also increase creatinine by increasing muscle amidohydrolase) is a recent advance. Creatininase is followed by breakdown. creatinase, sarcosine oxidase, and peroxidase reactions. URIC ACID Uric acid is the major product of nucleoprotein catabolism in Creatinine + H2o Creatine (Eq. 11-8) Creatinase humans and higher primates. The breakdown of adenine and gua-nine, purine nucleosides found in nucleic acids (DNA and RNA), results in the formation of uric acid. Adenine and guanine are Creatinase catabolized to xanthine, and uric acid is produced in the liver from Creatine + H2o Sarcosine + urea (Eq. 11-9) xanthine by the action of the enzyme xanthine oxidase. Xanthine oxidase Xanthine uric acid (Eq. 11-16 Sarcosine + H2o + o2 glycine + Sarcosine oxidase Formaldehyde + H2o2 (Eq. 11-10) Uricacid is formed from both exogenous (dietary) and endog-enous (produced within the body) nucleotides, but most uric acid is derivedfrom endogenous nucleic acids, cells breaking down, Peroxidase H2o2 + Reduced indicator and cells being replaced. Seventy percent of uric acid is excreted by the kidneys, and the remainder is degraded by bacteria in the oxidized indicator + H2o (Eq. 11-11) gastrointestinaltract.6  CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 267 In lower mammals, uric acid is further reduced by uricase to form allantoin, which is water soluble and mammals major prod-uct BOX 11-4 Hyperuricemia of purine catabolism. Primary Hyperuricemia Gout Uricase Uricacid Allantoin (Eq. 11-17) Idiopathic Secondary Hyperuricemia Cytotoxic chemotherapy Uric Acid Clinical Significance Radiation therapy (leukemia, lymphoma) Hyperuricemia is defined asa serum or plasma uric acid con-centration Malignancy (cancer) of 77.0 mg/dLin menand 76.0 mg/dLin women. Acute or chronic renal disease, renal failure Causes of hyperuricemia are divided into four categories: increased dietaryintake, overproduction of uric acid, underex-cretion Increased tissue catabolism/starvation of uric acid, and specific enzyme defects. Mostare caused Glycogen storage disease by a combination of overproduction and underexcretion of uric High purine diet acid. Ethanol abuse Primary gout is associated with overproduction and essential Toxemia of pregnancy hyperuricemia. It is aninborn error of metabolism found predomi-nantly Severe exercise in men30to 50yearsof ageandis 7times morecommonin menthan women. Symptoms include arthritis (pain, inflammation Poisons (lead) of the joints), nephropathy, and nephrolithiasis.7 Drug therapy (diuretics,barbiturates) The patient often presents withaninflamed bigtoe, the first Lesch-Nyhan syndrome metatarsophalangeal joint (MTP), usually the first joint affected in gout. Gout occurs when monosodium urate precipitates in supersaturatedbody fluids; depositsof uratesare responsiblefor Uric Acid Methodologies the symptoms. Acute attacks of gout can be precipitated by alco-hol, Thetwo most common uric acid methodologies are phosphotung-stic high-protein diets, stress, acute infection, surgery, and certain acid (Carraway) and uricase. Phosphotungstic acid (PTA) medications.Treatment of goutincludes a diet adequate but not measures the development of a blue color (tungsten blue) when high in protein, no alcohol, normal weight maintenance, and drug phosphotungstic acid is reduced by uric acidin an alkaline medium. therapy if required. Patients should avoid or limit organ meatsor Sodium carbonate is added to maintain the alkaline pH. meatswith a high purine content(e.g.,liver, kidney,salmon, had-dock, scallops, heart, herring, mussels, pheasant, partridge, yeast, Na2CO3 smelt,and sardines).8 Allopurinol is one of the medications that Uric acid + Phosphotungstic acid Allantoin + inhibits xanthine oxidase,resulting in decreasedlevels of serum CO2 + Tungsten blue (Eq. 11-18) and urine uric acid. Othercauses of increased uric acid productioninclude cyto-toxic Interference by substances that reduce PTA is similar to those chemotherapy and radiation therapy in cancer patients, which that affect the Jaffe creatinine reaction. They include endogenous results in increased cell destruction and increased endogenous compounds (glucose and ascorbic acid) and exogenous compounds nucleicacids. (acetaminophen, acetylsalicylic acid, and caffeine). Uricase is more Secondary hyperuricemia can be attributed to several causes, specific, does not require a protein-free filtrate, andis used by 99% including renal retention of uric acidin acute or chronic renal dis-ease of the labs. Uricase catalyzes the oxidation of uric acid to allantoin. and renalfailure. Toxemiaof pregnancy,rigorousexercise,poi-sons, and drug therapy are examples of other conditions associated with secondary hyperuricemia. Uric acid Uricase Allantoin + H2O2+ CO2 (Eq. 11-19) Enzyme deficiencies including Lesch-Nyhan syndrome can also result in hyperuricemia. In Lesch-Nyhan, a deficiency The decrease in absorbance is measured at 293 nm, which is a of hypoxanthine-guanine phosphoribosyl transferase (HGPRT) producesan elevated uric acid. It is an X-linked genetic disorder peak absorbance for uric acid and one at which allantoin does not absorb. The decrease in absorbance is thus inversely proportional characterized by mentalretardation, abnormal muscle movements, to the uric acid concentration; the lower the absorbance, the higher and behavioral problems such as pathological aggressiveness and self-mutilation. See Box 11-4 for a morecompletelist of condi-tionsthe uric acid concentration. A second indicator reaction utilizing peroxidase and a dye associated with hyperuricemia. (4-aminoantipyrene) is a second modification and the most com-mon Hypouricemia, definedasa uric acid 62.0 mg/dL,is much less common than hyperuricemia.It can besecondaryto severe automated method. hepatocellular disease, defective renal tubular reabsorption of uric acid (congenital or acquired, Fanconis syndrome, or Wilson dis-ease), Peroxidase H2O2+ 4@Aminoantipyrine Chromagen (Eq. 11-20 and uricosuric drugs(allopurinol).  268 CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN Creatinine Clearance MINI-CASE 11-1 Creatinine clearance (CrCl) is the most popular and practical John, a 54-year-old male, presented with severe pain in his methodfor estimatingthe GFR.It is easily measured,and exten-sive wrists and right toe. He had been steadily gaining weight data are available for all age groups. Creatinine is a very good over the last few years and was now about 50 pounds indicator of glomerularfiltration rate for three reasons. First,it is overweight. He reported that he drinks about a six pack freely filtered bythe glomeruli. Second,it is not reabsorbed bythe of beer every day. tubules to any significant extent. Third, creatinine is released into Clinically significant results on his chemistry profile the plasmaat a constantrate, resulting in constant plasmalevels include over 24 hours. Alinear decreasein creatinine clearance over time asrenal function fails has been documented for different diseases (e.g.,chronic glomerulonephritis). Reference Creatinine levels are measuredon serum and urine specimens. Range A 24-hour urine is usually collected. The CrCl is calculated using BUN 18 mg/dL 6.023.0 mg/dL the serum and urine creatininelevels andthe urine volume. Creatinine 0.7 mg/dL 0.61.4 mg/dL Uric acid 10.9 mg/dL 3.47.0 mg/dL U * V Clearance(X) = (Eq. 11-21) P His physician suggested that along with exercise, John should stop drinking or, at the very least, cut down on his alcohol intake and consider alow-purine diet. where 1. What is Johns probable diagnosis? U = urine concentrationin mg/dL, 2. What physical and laboratory findings led to your P = plasmaconcentrationin mg/dL, and diagnosis? V = urineflow in mL/minute(1440 min/24h) S 3. What can precipitate an attack? 24h volume (mL/day). 4. List five foods that John should limit on alow purine 1440 min/day diet. The volume of urine in mL/minute is calculated by dividing the 24h volume in mL/day by 1440 minutes/day, which results in Ascorbate oxidase may be added to minimize ascorbic acid mL/min. Plasma creatinine is inversely proportional to the clear-ance: interference, and potassium ferricyanide to minimize interference The higher the plasma creatinine, the lower the clearance. from bilirubin. Creatinine clearance has to be corrected to an adult body surface area (BSA) of 1.73 m2, which is especially important for Uric Acid Reference Range small or pediatric patients and obese patients. This can be done in The reference range for the uricase methodology is 3.57.2 mg/dL two ways: using the Dubois formula or a nomogram. The Dubois for males and 2.66.0 mg/dL for females. Uric acid is susceptible formula is to bacterial action, so specimens should be refrigerated. Increased hemolysis and bilirubin decrease uric acid using the peroxidase reaction. Uric acid is not affected by recent diet, but it can beinflu-enced SA(surface areain m2)= W(kg)0.425* H(cm)0.725 by a long-term purine-rich diet. * 0.007184 (Eq. 11-22) ANALYTICAL PROCEDURES The anomogram is much easier. See Appendix I. FOR ASSESSMENTOF The correction or normalization factor for BSA is added to GLOMERULAR FILTRATION the CrCl equation. Clearance Tests Renal clearance is defined as the rate at which the kidneys remove (U * V) 1.73 m2 * = a substance from the plasma or blood or a quantitative expression P BSA m2 (normalization factor) of the rate at which a substance is excreted by the kidneys in rela-tion CrCl (mL/minute/1.73 m2) (Eq. 11-23) to the concentration of the same substance in the plasma, which is usually expressed as mL cleared per minute. The best markers are freely filtered by the glomeruli, produced at a constant The corrected CrCl for a larger or obese individual will be rate, not reabsorbed or secreted by tubules, and present in stable lower because the normalization factor will be less than one. For a concentrations in the plasma, andthey have an inexpensive and pediatric patient or small adult, the corrected CrCl will beincreased rapid assay for detection.9 with a normalization factor greater than one  CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 269 those at risk for chronic kidney disease (CKD)for example, those MINI-CASE 11-2 with diabetes, hypertension, cardiovascular disease, or a family his-tory of kidney disease.10 The primary reasons are that GFR and A creatinine clearance was ordered on Jack, an obese creatinine clearance are not as accurate as using creatinine alone, patient with kidney disease. and creatinine is more often measured than urinary albumin. Also, Weight: 350 lb the Modification of Diet in Renal Disease (MDRD) Study equation Height: 5'4'' has been thoroughly validated and is superior to other methods of 24-hour urine volume: 1850 mL approximating GFR. It does not require weight or height variables because it is normalized to 1.73 m2 body surface area, which is the Plasma creatinine: 6.5 mg/dL acceptedBSAas discussed in the section on creatinineclearance.10 Urine creatinine: 120 mg/dL In patients 18 years of age and older, the MDRD equation BSA: 2.30 m2 is the best means currently available to use creatinine values as a measure of renal function. The equation has been validated in the 1. What is Jacks creatinine clearance? Caucasian and African American populations with impaired renal 2. Is the creatinine clearance low, normal, or elevated? function (eGFR 660 mL/min/1.73 m2)betweenthe agesof 18 3. What is the relationship between BSA and the cor-rected and 70. It requires four variables: serum or plasma creatinine, age CrCl? in years, gender, and race (African American or not).10 When the serum creatinine has been calibrated to be trace-able using an isotope dilution massspectrometry (IDMS) method, the following MDRD equation is recommended by the National The patient should not drink any caffeinated beverages(tea, Kidney Disease Education program:10 coffee)the day of the test and urinecollection. Theclearancecan When Scris in mg/dL (conventional units): be performed on a 4-, 12-, or 24-h urine with the important vari-able being a well-hydrated patient. The patient should drink 500 mL of water10 to 15 minutes beforethe test to ensure proper eGRF(mL/min/1.73 m2)= 175 * (Scr)-1.154* (Age)-0.203 * hydration and a urine flow rate of morethan 2 mL/min, whichis critical to an accurate CrCl. 0.742[if female] * 1.212[if African American] Thepatientsinstructions should stressthe importance of com-plete (Eq. 11-24) collection because the largest source of error is incomplete urine collection. The bladder should be completely emptied before timing is initiated. Vigorousexerciseand musclemass (as discussed A second equation for CKD-EPI is more accurate for under serum creatinine) can affect creatinine values. Proteinuria GFR 760 mL/min/1.73m2. also results in increased creatinine clearance. The reference range for males is 97137 mL/minute;for females, 88128 mL/minute. GFR = 141 * min(Scr/k,1)a* max(Scr/k,1)-1.209* Creatinine clearance decreases with age approximately 1 mL/min/ year. Increased CrCl is not clinically significant and probably an 0.993Age * 1.018 [if female] * 1.159 [if African American] error in specimencollection. DecreasedCrClindicates decreased (Eq. 11-25) GFR as a result of acute or chronic damage to the glomeruli. Mild renal impairment is indicated by a CrCl of 5079 mL/min; moderate, 1049 mL/min;andsevere,610 mL/min. where Scris serum creatininein mg/dL, CHECKPOINT! 11-2 k is 0.7 for females and 0.9 for males, ais -0.329 for femalesand -0.411for males, 1. Whyis creatinine clearance the most widely used test for estimating the glomerular filtration rate? min indicates the minimum of Scr/k or 1, and 2. List three reasons why creatinine is described as a maxindicates the maximumof Scr/k or 1. good indicator of the glomerular filtration rate. For estimating GFR from serum creatininein patients under 3. Whatis the most common source of error in calculat-ing age 18, the Bedside Schwartz equation should be used.11 the creatinine clearance? height (cm) eGFR = 0.41 * (Eq. 11-26) Scr(mg/dL) eGFR Manystudies haverecommended using an estimating or prediction equationeGFRto estimate glomerularfiltration rate from the Laboratories have elected to routinely report both the eGRF serum creatinine level in patients with chronic renal disease and and the creatinine  270 CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN Protein: Creatinine Ratio decreased in some renal diseases, which will be discussed later in this chapter. Proteinuria as determined by a 24h urine protein and creati-nine clearance is subject to several sources of error, including incompleteness of collection, as discussed in a previous section. b2@ Microglobulin A protein:creatinine ratio for urine collected over a shorter period b2@microglobulin (BMG) is a small, nonglycosylated protein found of time (e.g., 4h or random urine) has been determined as a result on the cell membrane of most nucleated cells, and it is present in of many studies to predict the presence of significant proteinuria. especially high levels in lymphocytes. b2@microglobulin is increased Normal protein excretion is 61009150 mg/24h, and creatinine in renal failure, and the plasma level of BMG is a good indicator excretion is fairly constant at 1520 mg of creatinine per kilogram of renal tubular function.16 It is used primarily to test for renal of body weightper day. Anormal protein:creatinineratio is 60.1 tubular function in renal transplant patients when decreased (100150 mg protein/10001500 mg creatinine). A metaanalysis tubular function indicates early rejection. BMG has been found of 16 studies by Priceet al.12concludedthat a protein:creatinine in some studies to be an earlier indicator of rejection than CR ratio performed on a random urine has a strong correlation with because it is not influenced by lean muscle mass or variations in 24h protein excretion and may be used to rule out the presence of daily excretion.17 (See Chapter 9 Amino Acidsand Proteinsfor addi-tional significant proteinuria as defined by a quantitative measurement of information.) the 24h protein excretion. Low-Molecular-Weight Proteins Inulin Clearance Low-molecular-weight proteins such as a1@microglobulin, Inulin, an endogenous, naturally occurring polysaccharide found in a2@microglobulin, b@trace protein, and cystatin C are cleared from artichokes, is the gold standard for measuring glomerular filtration the plasma by glomerular filtration and can be considered freely rate. Inulin is injected and measured in serum and urine over three filtered at the glomerular filtration barrier. hours. Major disadvantages are that, unlike creatinine, there is no These proteins are filtered and metabolized in the proxi-mal easily performed methodology for inulin, and it is an invasive pro-cedure convoluted tubules and eliminated in the urine. A disadvantage requiring injection of inulin. It also requires water loading of all of the proteins listed, except for cystatin C, is the influ-ence to stimulate diuresis, bladder catheterization to assure complete-ness of nonrenal factors, including inflammation and liver disease. of collection, and carefultiming of blood samples.13 Cystatin C, however, appears to be more specific for measuring the GFR than the previously discussed proteins and creatinine Cystatin C clearance. Cystatin C, a single-chain, nonglycosylated, low-molecular-weight protein synthesized by all nucleated cells, is a cysteine protein Urinalysis inhibitor. Its mostimportant characteristicssmall size and high A routine urinalysis may be the first indication of renal disease. isoelectric point (pI = 9.2)enable it to befreely filtered bythe The appearance (color, turbidity) of the urine can provide valuable glomeruli and catabolized in the proximal convoluted tubules. Cys-tatin clues. For example, an amber or darker yellow urine is indicative Cis produced at a constant rate, and serum concentrations are of a more concentrated urine or the presence of bilirubin or both, not affected by muscle mass,diet, race, age, or gender.14 pink or red is associated with hemoglobin or myoglobin, and a The most practical cystatin C methodologies for use in the white foam is suggestive of proteinuria. clinical laboratory are latex particleenhanced turbidimetric and The reagentstrip tests provide valuable information regarding nephelometric immunoassay. renal function. The cellulose pads on a plastic strip are impregnated Studies comparing cystatin C and creatinine have found with various reagents with as many as 10 pads or tests measuring that cystatin Cis superior to creatinine for the detection of renal different constituents on one strip. The reader is referred to several disease, and it is especially useful in detecting mild to moderate excellent urinalysis textbooks in the reference list at the end of this impairment of renal function. Serum cystatin Clevels have alinear chapter for more detailed information. association across the GFR range, including patients with GFR of The proteintest matutilizes tetrabromphenol blue andis based 60990 mL/min/1.73m2, which has been described as preclinical on the protein error of indicators. Proteinuria is often the first kidney disease.14 In a study by Villa et al.,15 of 25 patients with symptom of kidney disease. Many biological variables affect pro-tein renal dysfunction, five (20%) had elevated serum creatinine levels, excretion, including upright position, exercise, fever, heart fail-ure, whereas 19 (76%) had elevated cystatin C. and, of course, kidney disease. Urine protein is discussed in moredepthin Chapter9 AminoAcidsand Proteins. SCREENING FOR RENAL DISEASE Reagent strip methodologies that are more specific for albumin, myoglobin, and other low-molecular-weight Serum Protein proteins are available. Chromogenic reaction of albumin Severe renal disease is characterized by decrease in total protein, with bis@(3',3'@diiodo@4',4'@hydroxy@5',5'@dinitrophenyl)@3,4,5, especially the smaller-molecular-weight proteins such as albumin. 6@tetrabromosulonephthalein (DIDNTB) at pH 1.5 produces a Both serum total protein and albumin levels therefore will be measurable color reaction.1  CHAPTER 11 NoNPRoTEiN NiTRogEN ANd RENAl FuNCTioN 271 Hemoglobin testing is a