A high BUN-to-creatinine ratio is found in overproduction of urea nitrogen or in reduced excretion of urea nitrogen. The ratio is best used only as a rough guide, due to variability in results related to variability in diet and muscle mass.


BUN-to-Creatinine ratio =

= ((serum urea nitrogen in mg/dL) / (serum creatinine in mg/dL))




The normal range for a healthy person on a normal diet is 12-20, with most 12-16.




normal (12 – 20)

• normal renal function

• acute renal failure

• chronic renal failure

elevated (> 20)

• prerenal azotemia;

• postrenal azotemia;

• catabolic state, with reduced muscle mass;

• drug (tetracycline, glucocorticoid) effects;

• impaired renal function with urine reabsorption, for example with ureterocolostomy;

• impaired renal function with excess protein intake/production or with excess tissue breakdown, as may occur with GI bleeding, thyrotoxicosis, infection, Cushing's syndrome, surgery, burns, cachexia, high fever, or high protein diet.

• A ratio > 36 suggests upper GI bleeding.

decreased (< 10)

• renal dialysis

• acute tubular necrosis

• low protein diet or starvation

• syndrome of inappropriate ADH secretion

• pregnancy

• severe liver disease

• inherited hyperammonemia



• A person with low muscle mass or protein malnutrition may have a low serum BUN, and the ratio would be an unreliable indicator of renal status.


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