Renal & Electrolytes

BUN/Cr Ratio Calculator

BUN over creatinine; a high ratio points toward prerenal causes such as hypovolemia or GI bleeding.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

BUN / Cr

How it is used

• A ratio over 20:1 suggests a prerenal state, alongside FENa under 1% and urine sodium under 20.
• Upper GI bleeding raises BUN from digested blood; high-protein feeding, corticosteroids and catabolism raise it too.
• Liver failure and malnutrition lower BUN, and low muscle mass lowers creatinine, so the ratio can mislead.

Same formula as the app. Open it in the web app.

Related one-pagers

• GI Bleeding, Liver Failure and PancreatitisPeptic ulcer is the leading upper GI source, then varices, erosive disease, Mallory-Weiss tears and angioectasia. As little as 50–100 mL of blood can cause melena, and digested blood raises the BUN; a BUN/creatinine ratio above 30 favors an upper source (Srygley 2012).
• Electrolyte Imbalances
• ABG Interpretation in 6 Steps
• CRRT (Continuous Renal Replacement)

Related calculators

• Anion Gap
• Corrected Calcium
• Corrected Sodium (Hyperglycemia)
• Creatinine Clearance (Cockcroft-Gault)
• CRRT Effluent Dose (mL/kg/h)
• Fractional Excretion of Sodium (FENa)
• Fractional Excretion of Urea (FEUrea)
• Free Water Deficit

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.