BUN-to-Creatinine Ratio

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).

Interpretation

Increased Ratio (>10:1) with Normal Creatinine In

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

Increased Ratio (>10:1) with Elevated Creatinine In

Postrenal azotemia (BUN rises disproportionately more than creatinine) (e.g., obstructive uropathy)

Prerenal azotemia superimposed on renal disease

Decreased Ratio (<10:1) with Decreased BUN In

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)

Pregnancy

Decreased Ratio (<10:1) with Increased Creatinine In

Phenacemide therapy (accelerates conversion of creatine to creatinine)

Rhabdomyolysis (releases muscle creatinine)

Muscular patients who develop renal failure

Limitations

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)