Clinical Pathology: Kidney Function

Kidney Function

• The kidneys regulate the internal environment of the body through various functions, including:
  • Eliminating excess water and inorganic elements
  • Removing nonvolatile end products of metabolic activity
  • Retaining essential substances for normal bodily functions
  • Eliminating foreign toxic elements
  • Forming and excreting substances like hydrogen ions and ammonia

Renal Function Tests

• These tests assess kidney function and include:
  • Urine specific gravity and water deprivation tests
  • Estimation of non-protein nitrogen levels in the blood
  • Studies on the ability of the kidney to excrete certain dyes
  • Clearance concept tests
  • Other blood chemistry determinations (e.g., serum amylase, electrolytes, blood pH, cholesterol, and serum proteins)

Non-Protein Nitrogen Levels in Blood

• Non-protein nitrogen (NPN) refers to nitrogen-containing components in serum or plasma not associated with protein
• NPN includes:
  • Urea, creatinine, creatine, uric acid, ammonia, amino acids, and "undetermined nitrogen"
• NPN represents products of intermediary metabolism from both tissue and ingested protein

Blood Urea Nitrogen (BUN)

• Urea is the principal end product of protein catabolism, formed in the liver and excreted primarily by the kidneys
• Approximately 25-40% of filtered urea is reabsorbed in the tubules
• Indications for BUN testing:
  • Suspected decreased kidney function
  • Measuring peripheral perfusion of tissues in animals with hypovolemic shock or decreased blood pressure
  • Routine presurgical laboratory screening

Technique of BUN Tests

• BUN determinations are usually conducted on whole blood, plasma, or serum
• Anticoagulants containing nitrogen must be avoided; EDTA, heparin, sodium citrate, and sodium oxalate are acceptable
• Various laboratory techniques are used to estimate BUN concentration, including:
  • Chromatography
  • Mercury combining power
  • Nesslerization
  • Determination of Urease enzyme

Chromatography Technique

• Advantages:
  • Simplicity
  • No special chemicals required
  • Small amount of serum or plasma needed
• Principle of the test:
  • Chromatography paper is banded with different reagents, which react with the serum to produce a color change, indicating urea nitrogen concentrations

Interpretations of BUN Results

• If BUN concentration exceeds 35-45 mg/dl, glomerular filtration rate (GFR) is diminished
• Abnormal BUN concentrations may be caused by prerenal, primary renal, or postrenal factors
• Correlation can be made between the degree of BUN elevation and the severity of renal function impairment

Creatinine

• Creatinine is a nonprotein nitrogen substance formed during muscle metabolism of creatine and phosphocreatin
• It is excreted by glomerular filtration and provides an index of glomerular filtration
• Factors influencing creatinine concentration are similar to those for BUN, except:
  • Creatinine is not influenced by diet
  • Daily production of creatinine from muscle metabolism is relatively constant
  • Creatinine production is not easily influenced by catabolic factors affecting urea formation

Interpretations of Creatinine Concentration

• Similar to BUN interpretations, with elevations indicating decreased GFR and potential renal function impairment