Veterinary Clinical Biochemistry and Laboratory Diagnosis-II

KIDNEY FUNCTION TESTS - II

Blood Urea Nitrogen (BUN)

• The BUN test measures the amount of nitrogen contained in the urea
• Urea is a relatively nontoxic substance formed in the liver as a means of disposing of ammonia from protein metabolism.
• Urea is filtered by the glomerulus.
• The real concentration of urea is BUN x 2.14.
• Normal blood urea nitrogen is 8-25 mg/dL (2.9-8.9 mmol/L).
• Blood urea levels are sensitive indicators of renal disease, becoming elevated when renal function drops to around 25-50% of normal.
• By definition increased BUN is Azotemia.
• It is due either to increased protein catabolism or impaired kidney function.
  • Increased protein catabolism results from:
    • A really big protein meal (Kansas City steak, etc.)
    • Severe stress (myocardial infarction, high fever, etc.)
    • Upper GI bleeding (blood being digested and absorbed)
  • Impaired kidney function may be "prerenal", "renal", or "postrenal".
    • Prerenal azotemia results from underperfusion of the kidney:dehydration, hemorrhage, shock, congestive heart failure; glomerulonephritis is likely also to be "prerenal" if mild, since it comprmises renal blood flow more than tubular function.
    • Renal azotemia has several familiar causes:
      acute tubular necrosis, chronic interstitial nephritis, some glomerulonephritis, etc.
    • Postrenal azotemia results from obstruction of urinary flow:prostate trouble, stones, surgical mishaps, tumors.
• In acute renal failure, BUN increases around 20 mg/dL each day (*estimates vary; range of increase is 10-50 mg/dL daily).

Creatinine

• This breakdown product of creatine phosphate is released from skeletal muscle at a steady rate.(Only a small amount comes from meat in the diet).
• It is filtered by the glomerulus, and a small amount is also secreted into the glomerular filtrate by the proximal tubule. (hence at low GFR's, the usual reciprocal relationship breaks down and creatinine tends to underestimate how low the GFR has gotten).
• Creatinine is generally considered a somewhat more sensitive and specific test of renal function than BUN.
• Normal serum creatinine is 0.6-1.5 mg/dL (53-133 micromoles/L).

Other blood tests

• Measurement of the blood levels of other elements regulated in part by the kidneys can also be useful in evaluating kidney function. These include
  • Sodium,
  • Potassium,
  • Chloride,
  • Bicarbonate,
  • Calcium,
  • Magnesium,
  • Phosphorus,
  • Protein,
  • Uric acid,
  • Glucose.