Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (750 page)

   Limitations

   The pH of freshly drawn blood decreases on standing at a rate of 0.04–0.08 pH U/hour at 37°C, by approximately 0.03 U/hour at 25°C, but only 0.008 U/hour at 4°C.

BLOOD UREA NITROGEN (BUN)

   Definition

   Protein and nucleic acid catabolism results in the formation of urea and ammonia. Urea is synthesized mainly in the liver, and >90% is excreted through the kidneys.

   
Normal range:
7–23 mg/dL.

   Use

   Most widely used screening test for the evaluation of kidney function.

   Along with the serum creatinine, BUN levels aid in the differential diagnosis of prerenal, renal, and postrenal hyperuremia.

   Diagnosis of renal insufficiency: filtered freely in the glomerulus; ≤50% is reabsorbed.

   Assessment of glomerular function: A BUN of 10–20 mg/dL almost always indicates normal glomerular function.

   In chronic renal disease, BUN correlates better with symptoms of uremia than does serum creatinine.

   Provides evidence of hemorrhage into the upper GI tract.

   Assessment of patients requiring nutritional support for excess catabolism, for example, burns, cancer.