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Subject: BUN-to-Creatinine Ratio
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The BUN-to-creatinine ratio is used to differentiate prerenal and postrenal azotemia from renal azotemia. Because of considerable variability, it should be used only as a rough guide.
Normal range (usual range for most people on normal diet: 12–16).
Prerenal azotemia (e.g., heart failure, salt depletion, dehydration, blood loss) due to decreased GFR
Catabolic states with increased tissue breakdown
GI hemorrhage; a ratio ≥36 is reported to distinguish upper from lower GI hemorrhage in patients with negative gastric aspirate.
High protein intake
Impaired renal function plus
Excess protein intake or production or tissue breakdown (e.g., GI bleeding, thyrotoxicosis, infection, Cushing syndrome, high-protein diet, surgery, burns, cachexia, high fever)
Urine reabsorption (e.g., ureterocolostomy)
Patients with reduced muscle mass (subnormal creatinine production)
Certain drugs (e.g., tetracycline, glucocorticoids)
Selective increase in plasma urea (diuretic-induced azotemia) during use of loop diuretics
Postrenal azotemia (BUN rises disproportionately more than creatinine) (e.g., obstructive uropathy)
Prerenal azotemia superimposed on renal disease
Acute tubular necrosis
Low-protein diet, starvation, severe liver disease, and other causes of decreased urea synthesis
Repeated dialysis (urea rather than creatinine diffuses out of extracellular fluid)
Inherited deficiency of urea cycle enzymes (e.g., hyperammonemias—urea is virtually absent in blood)
SIADH (due to tubular secretion of urea)
Phenacemide therapy (accelerates conversion of creatine to creatinine)
Rhabdomyolysis (releases muscle creatinine)
Muscular patients who develop renal failure
DKA (acetoacetate causes false increase in creatinine with certain methodologies, resulting in normal or decreased ratio when dehydration should produce an increased ratio)
Cephalosporin therapy (interferes with creatinine measurement)