BUN/Creatinine Ratio Calculator

How the calculation works

The blood urea nitrogen (BUN) to serum creatinine ratio is a simple mathematical division:

Ratio = BUN (mg/dL) ÷ Serum Creatinine (mg/dL)

BUN and creatinine are filtered by the kidneys in different ways. Creatinine is filtered freely and not reabsorbed, while urea is reabsorbed back into the blood, particularly during dehydration. This difference in kidney filtration dynamics is what makes the ratio a useful diagnostic tool.

Interpreting the ratio, and who publishes each line

• < 10:1 (Low ratio): Associated with liver disease, low protein intake, SIADH, pregnancy, or intrinsic renal damage. The cut-off is the acute tubular injury column of the Merck Manual Professional table of urinary diagnostic indices, adapted from Miller TR et al., Ann Intern Med 1978. StatPearls uses < 20:1 for the same pattern.
• 10:1 to 20:1 (this tool's middle band): A rule of thumb, and the one figure on this page with no primary source behind it. Clinical Methods publishes a single expected ratio of about 10:1 and says it applies best in moderate to advanced renal failure, which is not the same claim.
• > 20:1 (High ratio): Most commonly caused by dehydration (pre-renal azotemia), high protein intake, gastrointestinal hemorrhage, or congestive heart failure. This is the prerenal cut-off from the same Merck table; a different Merck page puts the abnormal threshold at > 15.

What moves the ratio without the kidneys changing

Clinical Methods tabulates the extrarenal factors, and this is the reason a single ratio is a weak thing to reason from. Urea production rises with a high-protein diet, with gastrointestinal bleeding, with catabolic states such as fever or infection, and with glucocorticoids and most tetracyclines. It falls with a low-protein diet, with malnutrition or starvation, and with impaired liver metabolism. Creatinine is largely unaffected by gastrointestinal bleeding or by fever and steroids, but it tracks lean body mass, declines with age as muscle mass does, and rises after eating cooked meat.

The same chapter states the practical consequence plainly: at all stages of renal insufficiency the serum creatinine is a much more reliable indicator of renal function than the BUN, because the BUN is far more likely to be affected by dietary and physiologic conditions unrelated to the kidneys. It gives the example of patients with congestive heart failure and intact kidneys who commonly present with a BUN of 50 to 70 mg/dL and a creatinine below 1.2 mg/dL.

Two further limits worth knowing. Merck notes that these urinary diagnostic indices are not reliable in nonoliguric states, which covers most people using a calculator. And pregnancy shifts the baseline: Clinical Methods notes that a BUN of 15 mg/dL would represent significantly impaired function for a woman in the thirtieth week of gestation, whose expected BUN is 5 to 7 mg/dL.