Calculate the BUN/Creatinine ratio to differentiate prerenal, intrinsic renal, and postrenal causes of kidney dysfunction. Includes eGFR and differential diagnosis.
The BUN/Creatinine ratio is one of the most commonly used laboratory calculations in medicine for differentiating the cause of elevated kidney function markers. Blood urea nitrogen (BUN) and serum creatinine are both waste products filtered by the kidneys, but they are affected by different physiological processes, making their ratio a valuable diagnostic tool.
BUN is produced from protein metabolism in the liver and is freely filtered by the kidneys but partially reabsorbed in the tubules—particularly when urine flow is slow (as in dehydration). Creatinine is produced at a relatively constant rate from muscle metabolism and is freely filtered with minimal reabsorption. This difference is the key to the ratio's diagnostic utility.
A ratio greater than 20:1 suggests a prerenal cause (dehydration, heart failure, GI bleeding) where reduced kidney perfusion leads to increased urea reabsorption while creatinine clearance is relatively preserved. A ratio of 10–20:1 is normal or consistent with intrinsic renal disease. A ratio below 10:1 suggests low urea production (liver disease, malnutrition) or conditions that disproportionately raise creatinine (rhabdomyolysis). This calculator computes the ratio, provides differential diagnosis guidance, and estimates GFR using the CKD-EPI 2021 equation.
The BUN/Creatinine ratio calculator provides instant differential diagnosis guidance for kidney dysfunction, helping clinicians quickly distinguish between prerenal, intrinsic renal, and other causes. It integrates eGFR calculation and provides clinical context for interpretation. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation.
BUN/Creatinine Ratio = BUN (mg/dL) ÷ Serum Creatinine (mg/dL). Normal ratio: 10–20:1. eGFR (CKD-EPI 2021): 142 × min(Cr/κ, 1)^α × max(Cr/κ, 1)^−1.200 × 0.9938^age × (1.012 if female).
Result: Ratio = 33.3:1 — Elevated ratio suggesting prerenal azotemia
BUN 40 / Creatinine 1.2 = 33.3:1. This elevated ratio with relatively preserved creatinine suggests a prerenal cause such as dehydration, heart failure, or GI bleeding. The disproportionate BUN rise occurs because urea reabsorption increases when kidney perfusion drops.
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Most mistakes come from mixed standards, rounding too early, or misread labels. Recheck final values before use. ## Practical Notes
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Normal is 10:1 to 20:1. Values above 20:1 suggest prerenal causes, while values below 10:1 suggest intrinsic renal disease or low BUN production.
Digested blood in the GI tract is broken down into amino acids, which are metabolized to urea in the liver. This protein load raises BUN disproportionately while creatinine remains stable, producing ratios often exceeding 30:1.
Yes. Corticosteroids increase protein catabolism (raising BUN). Trimethoprim and cimetidine inhibit creatinine secretion (raising creatinine). High-dose diuretics can cause prerenal azotemia.
In AKI, a high ratio (>20:1) suggests a reversible prerenal cause amenable to fluid resuscitation. A normal ratio with elevated absolute values suggests intrinsic renal damage (ATN) that may need different management.
Fractional excretion of sodium (FENa), urine osmolality, urine sodium, and urine microscopy complement the BUN/Cr ratio in differentiating prerenal from intrinsic renal failure. Use this as a practical reminder before finalizing the result.
Yes. Very muscular patients have higher baseline creatinine, which lowers the ratio. Elderly or cachectic patients have low creatinine, which can falsely elevate the ratio.