BUN Creatinine Ratio Calculator

BUN Creatinine Ratio Calculator for Nephrology. A ratio above roughly 20:1 (some use greater than 20 to 25) suggests a prerenal cause of azotemia, an upper GI bleed contributing a nitrogen load, or high urea production; a ratio of about 10:1 to 15:1 is normal and, when creatinine is elevated, is typical of intrinsic renal disease such as acute tubular necrosis; a ratio below 10:1 suggests low urea generation (low protein intake, advanced liver disease, or SIADH) or a postdialysis state. In suspected GI bleeding, a markedly elevated ratio in a patient with no renal impairment and normal fluid status increases the likelihood of an upper (rather than lower) source, because absorbed blood proteins are converted to urea.

How this calculator works

The ratio divides serum blood urea nitrogen (BUN, mg/dL) by serum creatinine (mg/dL); both are entered in conventional US units. Because urea is freely filtered and then variably reabsorbed along the nephron (reabsorption rises when tubular flow slows) while creatinine is filtered and largely not reabsorbed, the ratio behaves as a crude index of renal tubular urea handling and of urea generation. Outside the US, urea reported in mmol/L must be converted (urea mg/dL = urea mmol/L x 2.8, and BUN = urea mg/dL x 0.467) before the ratio is comparable to the familiar cutoffs.

When to use this calculator

Use it as a fast, no-cost triage step when azotemia is present: to sort a rising creatinine into prerenal versus intrinsic (particularly acute tubular necrosis) patterns, and to raise or lower suspicion for occult upper gastrointestinal bleeding in a patient with anemia or melena but no localizing findings. It is an adjunct only. Do not rely on it to diagnose or exclude AKI on its own, and do not apply the usual bands to patients on hemodialysis, those with very low muscle mass or cirrhosis, or anyone on high-dose corticosteroids or a high-protein regimen, where urea generation is disconnected from perfusion.

Inputs used

  • Blood urea nitrogen
  • Serum creatinine

Clinical interpretation

A ratio above roughly 20:1 (some use greater than 20 to 25) suggests a prerenal cause of azotemia, an upper GI bleed contributing a nitrogen load, or high urea production; a ratio of about 10:1 to 15:1 is normal and, when creatinine is elevated, is typical of intrinsic renal disease such as acute tubular necrosis; a ratio below 10:1 suggests low urea generation (low protein intake, advanced liver disease, or SIADH) or a postdialysis state. In suspected GI bleeding, a markedly elevated ratio in a patient with no renal impairment and normal fluid status increases the likelihood of an upper (rather than lower) source, because absorbed blood proteins are converted to urea.

Worked example

A dehydrated patient presents with BUN 60 mg/dL and creatinine 1.2 mg/dL, giving a ratio of 50. A value this high (well above 20) points strongly to a prerenal state with avid tubular urea reabsorption, or to a large urea load from digested blood in the gut, rather than to intrinsic tubular injury; the next steps are a volume challenge and a look for a GI source. Contrast a patient with BUN 28 and creatinine 2.8 (ratio 10): the low ratio with a high creatinine fits established acute tubular necrosis.

Limitations and safety notes

The ratio is neither sensitive nor specific and should never override urine electrolytes, the fractional excretion of sodium or urea, urine microscopy, or imaging. It is misleading whenever urea and creatinine are driven independently: corticosteroids, tetracyclines, GI bleeding, catabolic states, and high-protein feeds raise urea without a perfusion change, while low muscle mass, malnutrition, cirrhosis, and low-protein diets lower the denominator or the numerator out of step with true kidney function. Diuretics and osmotic agents alter urea reabsorption, and dialysis clears urea faster than creatinine, so post-treatment ratios are uninterpretable.

Frequently asked questions

Is BUN/creatinine ratio the same as urea/creatinine ratio?

No. BUN measures only the nitrogen fraction of urea, so it is about 0.47 times the urea mass. A serum-urea-to-creatinine ratio (used in much of the world) runs roughly 2 times higher than the BUN-based ratio, so the two use different cutoffs. Always confirm which quantity your lab reports before applying the 20:1 threshold.

Does a high ratio confirm dehydration?

No. It only raises suspicion. A ratio above 20 is consistent with a prerenal, volume-depleted state, but corticosteroids, GI bleeding, and a high-protein load produce the same pattern with normal volume status. Confirm with the clinical exam, urine sodium or fractional excretion of urea, and the response to a fluid challenge.

How does GI bleeding raise the ratio?

Hemoglobin and other blood proteins entering the gut are digested and absorbed as amino acids, which the liver converts to urea. This raises BUN while creatinine stays flat, so an upper GI bleed often pushes the ratio well above 20 in a patient with otherwise normal kidneys.

Why can the ratio be normal in acute tubular necrosis?

In established ATN the injured tubules lose their capacity to reabsorb urea, so urea and creatinine rise together. The ratio therefore stays around 10:1 to 15:1 even as both values climb, which is why a normal ratio with a high creatinine points toward intrinsic injury rather than a prerenal cause.

References

  • Srygley FD, Gerardo CJ, Tran T, Fisher DA. Does this patient have a severe upper gastrointestinal bleed? JAMA. 2012;307(10):1072-9. PMID: 22416103.
  • Manoeuvrier G, Bach-Ngohou K, Batard E, Masson D, Trewick D. Diagnostic performance of serum blood urea nitrogen to creatinine ratio for distinguishing prerenal from intrinsic acute kidney injury in the emergency department. BMC Nephrol. 2017;18(1):173. PMID: 28545421.
  • Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney International Supplements. 2012;2(1):1-138.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 19, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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