BUN-to-Creatinine Ratio Calculator

The page accepts BUN mg/dL or whole urea mmol/L and creatinine mg/dL or µmol/L from one specimen or a documented paired time. It normalizes both analytes to mg/dL, divides the unrounded values, and reports the conventional dimensionless BUN/Cr ratio with a normalized audit trail.

It does not calculate eGFR, measured GFR, creatinine clearance, FENa, FEUrea, kidney-disease stage, an AKI cause, or a GI-bleeding location.

Urea is the complete molecule, whereas BUN reports only the nitrogen mass contained in urea. A urea result in mmol/L therefore cannot be compared with a BUN result in mg/dL by changing only the unit label. The calculator performs a real conversion before division.[1, 3]

1. Identify whether the numerator is BUN mg/dL or whole urea mmol/L.
2. When needed, multiply urea mmol/L by 2.80112 to obtain BUN mg/dL.
3. Identify whether creatinine is reported in mg/dL or µmol/L.
4. When needed, divide creatinine µmol/L by 88.4 to obtain mg/dL.
5. Divide the full-precision normalized values without intermediate rounding.
6. Apply display rounding only after the raw ratio has been calculated.

BUN and creatinine must describe the same clinical state. Fluids, perfusion, catabolism, and changing kidney function can alter either analyte, so unrelated collection times may produce a ratio with no useful paired meaning. Editing a value or changing a unit clears confirmation and the prior result as a software safety measure.[3, 5]

BUN/creatinine combinations such as 20/1 and 40/2 produce the same ratio, although their absolute analytes and clinical contexts differ. The ratio cannot replace the original values, their trends, eGFR, urine output, or a complete assessment. The result card therefore retains both normalized analytes in its audit details.

A ratio greater than 20 has long appeared as a prerenal teaching clue, but about half of hospitalized patients with AKI in the Uchino study had a ratio above 20, and the study did not support BUN/Cr as a reliable prerenal-azotemia marker. This page labels neither side of 20 as prerenal, intrinsic, ATN, positive, or negative.[5]

AKI-cause assessment may require the creatinine trajectory, urine output, urinalysis, medicines, hemodynamics, obstruction assessment, and other evidence. BUN and creatinine also have different non-GFR determinants. The calculator does not distinguish prerenal, intrinsic, postrenal, or ATN causes.[4, 5]

A 2026 meta-analysis included 17 studies. A threshold above 30 had 38.8% sensitivity and 89.3% specificity; the meta-analysis optimal threshold of 22 had 66.2% sensitivity, 71.0% specificity, and AUC 0.740. The authors described moderate diagnostic accuracy. These aggregate study estimates are not a universal bedside rule, so this page applies none of the 22 or 30 cutoffs to an individual result.[8]

Machlab reported limited sensitivity: an elevated ratio may add evidence for an upper source in a selected population, while a lower result has limited exclusion value. This calculator cannot rule out upper GI bleeding or decide whether endoscopy is needed.[6, 8]

Russ reported AUROC values of about 0.69 with normal renal function, 0.56 with CKD, and 0.54 with AKI. In the AKI group, the difference between upper- and lower-bleeding ratios was not significant. Renal dysfunction therefore weakens localization performance, and this page never infers a bleeding source.[7]

• Dietary protein and gastrointestinal blood absorption.
• Fever, infection, burns, or another catabolic state.
• Glucocorticoids.
• Liver urea synthesis, malnutrition, or low protein intake.
• Perfusion, hydration, heart failure, or shock.
• Urinary obstruction and altered urea reabsorption.

[3]

• Muscle mass, frailty, muscle wasting, amputation, or paralysis.
• Meat or creatine intake.
• Medicines that affect secretion or measurement.
• Jaffe and enzymatic assay differences and interference.
• Unstable renal function or AKI.
• Rapid change that prevents a stable filtration interpretation.

[4]