For healthcare professionals
BUN to Creatinine Ratio Calculator
The BUN to creatinine ratio compares blood urea nitrogen with serum creatinine. In acute kidney injury, a high ratio is the pattern described for a prerenal cause, and a low ratio was linked to acute interstitial nephritis in a biopsy study. This tool is written for clinicians and is not a diagnosis.
Blood test values
Use BUN and creatinine from the same blood sample, both in mg/dL. Nothing you enter leaves this page.
What you will see
The BUN to creatinine ratio and whether it is over 20, 12 or less, or between.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
Both readings come from Lopez Giacoman S and colleagues, BMC Nephrology 2023, open access (CC BY 4.0): the over 20 prerenal pattern from its discussion of earlier work, and the 12 or less cut-off for acute interstitial nephritis from its own results. The tests check the ratio, both cut-offs at their exact values, and that a value near a cut-off is never shown as the cut-off.
Validated in
Lopez Giacoman 2023: 42 hospitalized adults (mean age 45) with acute kidney injury who had a kidney biopsy, in a prospective study; 19 had acute interstitial nephritis. The over 20 pattern is a general description, not a cut-off validated in a cohort here.
Limits
The ratio is a simple quotient, not a validated risk score. The 12 or less cut-off comes from one small study, and its authors call for validation in larger cohorts. The study reports the best cut-off as lower than 12 in its results and as 12 or less in its conclusion; the tool uses 12 or less. Creatinine production is directly related to muscle mass, which also moves the ratio. The tool takes US units (mg/dL) only.
The formula
BUN to creatinine ratio = blood urea nitrogen (mg/dL) / serum creatinine (mg/dL).
Worked example: a BUN of 40 mg/dL and a creatinine of 1.6 mg/dL give 40 / 1.6 = 25, over 20.
Reading the result
- Over 20: urea is filtered like creatinine but is also reabsorbed in the tubules; in hemodynamic (prerenal) acute kidney injury, antidiuretic hormone drives preferential urea reabsorption, resulting in a ratio over 20 (Lopez Giacoman 2023). Lima and Macedo also note that urea reabsorption raises the BUN to creatinine ratio (Lima 2018).
- 12 or less: in 42 adults with acute kidney injury and a kidney biopsy, the ratio was the only variable associated with acute interstitial nephritis; the best cut-off had a sensitivity of 81%, a specificity of 83% and an area under the curve of 0.73 (Lopez Giacoman 2023). The same paper also reports, for a ratio under 12 (12 or less in its abstract), a sensitivity of 76%, a specificity of 81%, a positive predictive value of 81% and a negative predictive value of 76% (Lopez Giacoman 2023). The same paper cites a ratio under 10 identifying acute interstitial nephritis in Mesoamerican nephropathy (Lopez Giacoman 2023).
Values over 12 and up to 20 have no published reading in these sources. Exactly 20 is not over 20, and exactly 12 counts as 12 or less.
Related tools
The FENa calculator and the FEUrea calculator use urine values to separate transient from persistent acute kidney injury. To estimate kidney function, see the eGFR calculator. Other bedside scores are listed under For healthcare professionals.
Sources
- Lopez Giacoman S, Gonzalez Fuentes C, Robles Davila J, Soto Abraham MV, Roman Acosta S, Chavez Iniguez J, et al. A low BUN/creatinine ratio predicts histologically confirmed acute interstitial nephritis. BMC Nephrol. 2023;24:75. doi:10.1186/s12882-023-03118-0. PMID 36967386. Open access (CC BY 4.0). Results (Youden cut-off, sensitivity, specificity, AUC), Discussion (ratio over 20 in prerenal AKI) and Conclusion. pmc.ncbi.nlm.nih.gov/articles/PMC10041724/
- Lima C, Macedo E. Urinary biochemistry in the diagnosis of acute kidney injury. Dis Markers. 2018;2018:4907024. doi:10.1155/2018/4907024. PMID 30008975. Open access (CC BY). Table 1 (formula; under 1% transient, over 1% persistent AKI) and text (Espinel; glomerulonephritis; obstruction; fractional excretion of urea). pmc.ncbi.nlm.nih.gov/articles/PMC6020498/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.