BUN : creatinine ratio

Calculate the ratio of blood urea nitrogen to serum creatinine, a supportive indicator of volume status and the pattern of renal impairment.

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What is this for?

Urea is reabsorbed with sodium and water in the proximal tubule while creatinine is not, so in volume depletion urea rises disproportionately. A ratio above roughly 20:1 supports a prerenal picture, whereas an intrinsic renal injury typically keeps the ratio nearer 10–15:1.

How to use it

Enter the blood urea nitrogen in mg/dL. If your laboratory reports urea rather than BUN, convert first — the result panel shows the SI equivalent to help you check.
Enter the serum creatinine, choosing the reported unit.
Read the ratio and its pattern band, then interpret alongside the clinical picture.

Worked example

A patient admitted with several days of vomiting has a BUN of 40 mg/dL and a creatinine of 1.4 mg/dL.

Answer: 40 ÷ 1.4 = 28.6:1. Above the 20:1 threshold, consistent with a prerenal pattern from volume depletion.

Clinical pearls & pitfalls

BUN and urea are not the same number. BUN in mg/dL multiplied by 2.14 gives urea in mg/dL; BUN in mg/dL multiplied by 0.357 gives urea in mmol/L. Many international laboratories report urea, so check before calculating.
A gastrointestinal bleed raises BUN by delivering a large protein load to the gut, producing a high ratio without volume depletion. It is one of the most important non-prerenal causes of a raised ratio.
Corticosteroids and a high-protein diet raise BUN; liver failure and malnutrition lower it. All of these shift the ratio without any change in renal perfusion.
The ratio is least useful in the very patients where volume status is hardest to judge — those with cirrhosis, heart failure, or on diuretics.

Assumptions & limitations

Not diagnostic. It is one supportive data point among the history, examination, urine output, urine sodium, and the creatinine trend.
Confounded by gastrointestinal bleeding, corticosteroids, protein intake, catabolic states, liver disease, and malnutrition.
The 20:1 threshold is a convention with no sharp physiological boundary; values near it should not be over-interpreted.

References

Rachoin JS, et al. The fallacy of the BUN:creatinine ratio in critically ill patients. Nephrol Dial Transplant. 2012;27(6):2248-2254.
Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2:1-138.