BUN/Creatinine Ratio Calculator

Blood urea nitrogen (BUN) and creatinine are different blood measurements. The conventional BUN/creatinine ratio divides BUN by creatinine only after both values have been normalized to compatible mg/dL units. The result is a contextual laboratory relationship, not a direct measurement of GFR and not eGFR or creatinine clearance.[3]

It also does not diagnose AKI, determine its cause, or identify a gastrointestinal bleeding location. The original BUN and creatinine values remain necessary because the same ratio can arise from very different analyte concentrations.

For the separate sodium, glucose and BUN calculation, see the serum osmolality calculator.

Select the reporting unit for each paired analyte. The frozen runtime normalizes whole urea and SI creatinine when needed, divides the full-precision normalized values, and rounds only the displayed result.

Urea is the complete molecule, whereas BUN reports only the nitrogen mass contained in urea. A UK/SI whole-urea result in mmol/L therefore cannot be inserted directly into the conventional BUN/Cr formula or treated as BUN by changing only the unit label. This calculator uses the fixed runtime conversion of urea mmol/L × 2.80112 before division; creatinine µmol/L is separately divided by 88.4.[1, 3]

A higher ratio means BUN is high relative to creatinine. Possible contexts include lower renal perfusion or volume depletion, gastrointestinal blood or protein absorption, high protein intake, catabolism, glucocorticoids, and heart-failure or shock physiology. Low creatinine from low muscle mass can also inflate the quotient.[3, 4]

Dehydration or another volume-depletion state can raise the ratio, but a high BUN/creatinine ratio does not diagnose dehydration. Interpretation still depends on both absolute analytes, history, volume assessment, kidney function, trends, and other laboratory information.

A ratio greater than 20 remains listed as a prerenal pattern clue in a current Merck educational table. That is static teaching context, not a universal diagnostic rule and not a classification applied by this calculator.[7]

In Uchino's study of 20,126 hospitalized patients, 3,641 had AKI and about half of those AKI patients had a ratio above 20; the findings did not support BUN/Cr as a marker of prerenal azotemia. In a separate 2017 emergency-department study of 1,103 patients, the ROC AUC was 0.5 and BUN/Cr did not discriminate prerenal from intrinsic AKI. Cause assessment requires evidence beyond this quotient.[5, 6]

About 10–20:1 is commonly quoted as adult teaching or reference context, but it is not a universal QuickMedCalc normal band. Laboratory methods, absolute analytes, age and body composition, diet, catabolism, liver urea synthesis, hydration or perfusion, and kidney function can all change the relationship. This page therefore does not label a result normal, high, or low.[3, 8]

A lower ratio may reflect lower BUN, higher creatinine, or both. Possible contexts include lower protein intake, reduced urea synthesis in advanced liver dysfunction, greater creatinine production or muscle injury, dialysis-related differences, and other clinical or laboratory factors. It does not automatically mean intrinsic kidney disease, ATN, liver disease, or another diagnosis.[3]

Is there a dangerously low BUN/creatinine ratio?

There is no universal ratio-only “dangerously low” threshold. Clinical importance depends on the actual BUN, actual creatinine, kidney function, symptoms, trends, and underlying condition rather than the quotient alone.

A high-normal BUN divided by a low-normal creatinine can produce a higher ratio even when each value remains within the reporting laboratory's own interval. Low muscle mass and a correspondingly low creatinine can amplify this effect. An isolated ratio cannot prove dehydration, kidney disease, or gastrointestinal bleeding, and this page does not collect or assume laboratory reference limits.[3, 4]

A 2026 systematic review and meta-analysis included 17 studies. At a threshold above 30, pooled sensitivity was 38.8% and specificity 89.3%. Its optimal threshold of 22 had 66.2% sensitivity, 71.0% specificity, and AUC 0.740; the authors described moderate diagnostic accuracy. These population estimates do not independently locate bleeding, and a lower ratio does not exclude an upper source.[9, 11]

Renal function also changes performance. Russ reported AUROC values of 0.69 with normal renal function, 0.56 with CKD, and 0.54 with AKI; in the AKI group, upper- versus lower-source ratios did not differ significantly. This calculator therefore applies no GI-localization cutoff.[10]

BUN/urea and creatinine should represent the same blood sample or a documented paired collection time. Unrelated dates or changing clinical states can create a quotient that does not describe one useful context.[3, 5]