About the BUN/Creatinine Ratio
Blood urea nitrogen (BUN) and creatinine are both waste products filtered out of the blood by the kidneys. BUN comes from the breakdown of protein, while creatinine comes from normal muscle metabolism. Because the two are cleared by the kidneys at somewhat different rates and are affected by different non-kidney factors, comparing them as a ratio — BUN ÷ creatinine, both expressed in mg/dL — can help clinicians distinguish between different causes of an abnormal kidney panel, such as dehydration versus intrinsic kidney damage. This calculator performs that division for you and converts SI-unit lab values (mmol/L urea nitrogen, µmol/L creatinine) to mg/dL first, since the standard reference range assumes mg/dL for both.
How the ratio is calculated
Enter your BUN and creatinine values along with the units your lab report used. If you select mmol/L for BUN, the calculator multiplies by 2.8 to convert to mg/dL (1 mmol/L urea nitrogen ≈ 2.8 mg/dL). If you select µmol/L for creatinine, it divides by 88.4 to convert to mg/dL (88.4 µmol/L = 1 mg/dL). The calculator then divides the converted BUN by the converted creatinine to produce the ratio, and compares it against the widely used 10:1 to 20:1 normal range.
Interpreting a high or low ratio
- Ratio above 20:1 ("high" or prerenal pattern): often reflects reduced blood flow to the kidneys rather than kidney damage — common triggers include dehydration, heart failure, low blood pressure, a high-protein diet, corticosteroid use, or gastrointestinal bleeding (digested blood adds extra nitrogen). Ratios above roughly 30:1 are classically associated with GI bleeding.
- Ratio below 10:1 ("low"): can reflect liver disease (less urea production), a low-protein diet or malnutrition, pregnancy, SIADH, or rhabdomyolysis, where muscle breakdown raises creatinine faster than urea.
- Reference ranges vary by lab and assay method, so your lab's own printed range takes precedence over any general guideline, and a single reading is a snapshot — trends across repeated tests are usually more informative than one isolated value.
Important disclaimer
This tool provides an educational estimate only and does not constitute medical advice or a diagnosis. It cannot account for your full clinical picture, medications, or hydration status. Always review your BUN, creatinine, and ratio results with a qualified healthcare provider before drawing conclusions or making treatment decisions.