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BUN Creatinine Ratio Calculator
The BUN/creatinine ratio is BUN in mg/dL divided by serum creatinine in mg/dL. Many teaching files call a ratio above 20 a prerenal pattern. That pattern is neither sensitive nor specific. Use it only with volume status, urine studies, and the clinical course.
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What the BUN/creatinine ratio is
Blood urea nitrogen (BUN) measures nitrogen from urea, the liver’s waste product from protein metabolism. Creatinine measures a muscle breakdown product.
Dividing BUN by creatinine, both in mg/dL, produces a dimensionless ratio used in teaching files to sketch a “prerenal” versus “intrinsic” pattern. The ratio is not a diagnosis of dehydration, gastrointestinal bleeding, or acute tubular injury. Both analytes rise when GFR falls. The ratio only asks whether urea rose out of proportion to creatinine.
If the laboratory reports urea in mmol/L instead of BUN in mg/dL, convert first. BUN (mg/dL) ≈ urea (mmol/L) × 2.8. Dividing urea mmol/L by creatinine mg/dL without conversion yields a meaningless number.
Formula and a worked ratio
Ratio = BUN (mg/dL) / creatinine (mg/dL). Example: BUN 28 and creatinine 1.1 gives 25.5.
Teaching notes often flag values above 20. That flag is a prompt to think about reduced kidney perfusion, high urea generation, or both. It is not a treatment algorithm. Urea 10 mmol/L corresponds to BUN about 28 mg/dL. Creatinine 97 µmol/L is about 1.1 mg/dL (97/88.4). The ratio is again about 25.
Many adult comments list roughly 10 to 20 as a common range when both tests are in mg/dL. Ranges differ by laboratory. Use the ratio with the absolute values. A ratio of 20 with creatinine 4.0 mg/dL is not reassuring.
Worked BUN/creatinine ratios
| BUN | Creatinine | Ratio | Teaching label only |
|---|---|---|---|
| 14 mg/dL | 1.0 mg/dL | 14 | Common range |
| 28 mg/dL | 1.1 mg/dL | 25 | Often called prerenal pattern |
| 8 mg/dL | 1.2 mg/dL | 6.7 | Low ratio pattern |
How to use the BUN/creatinine calculator
Enter BUN or urea with the matching unit, then serum creatinine with its unit.
Read the ratio as a laboratory relationship, then go back to volume status, medicines, urine output, and the trend in each analyte. Use paired values from the same blood draw. A BUN from admission and a creatinine from hospital day 3 do not describe either day.
KDIGO AKI staging uses creatinine rise and urine output, not this ratio. Do not delay fluids, obstruction imaging, or potassium treatment to compute a ratio.
When the ratio is high
Reduced kidney perfusion (true or effective volume depletion, heart failure, sepsis) increases urea reabsorption and can raise the ratio.
Gastrointestinal bleeding dumps protein into the gut and raises urea generation. Corticosteroids and high protein intake do the same. A high ratio with a normal creatinine can still be bleeding or high protein intake rather than “dehydration.” Orthostatic vitals, hemoglobin trend, stool, and history outperform the ratio.
Obstruction can produce any ratio. Post-renal injury is diagnosed with ultrasound and bladder scan, not with this arithmetic.
When the ratio is low
Low protein intake, advanced liver disease (less urea synthesis), pregnancy, and some intrinsic renal injury patterns can lower the ratio.
Rhabdomyolysis raises creatinine from muscle release and can drop the ratio. A low ratio does not prove acute tubular necrosis. Urine microscopy, FeNa or FeUrea in selected oliguric patients, and the clinical course do that work. This calculator does not compute FeNa.
Very low BUN should trigger a look at nutrition and liver function rather than a celebration of kidney health.
Processes that move BUN and creatinine differently
| Process | Typical effect on ratio | Better confirming test |
|---|---|---|
| Volume depletion | Often higher | Exam, orthostatics, response to fluid |
| GI bleeding | Higher (urea generation) | Hemoglobin, stool, endoscopy plan |
| Low protein intake | Lower | Diet history, albumin, urea trend |
| Rhabdomyolysis | Lower (creatinine up) | CK, urine dip, potassium |
| Advanced cirrhosis | Variable, often lower urea | Liver tests, measured GFR methods |
How the laboratory measures the inputs
BUN is usually a kinetic urease method. Creatinine in modern labs is IDMS-standardized enzymatic or compensated Jaffe.
Jaffe methods still have interferences (ketones, some cephalosporins). If the ratio looks impossible, ask whether the creatinine method changed. Hemolysis, delayed processing, and mixed-up tubes produce garbage ratios. Recollect rather than interpreting a hemolyzed sample.
SI laboratories may report urea, not BUN. Read the analyte name, not only the number.
Acute kidney injury context
KDIGO AKI stage 1 includes a creatinine rise of 0.3 mg/dL in 48 hours or 1.5 to 1.9 times baseline in 7 days, or oliguria. The BUN/creatinine ratio is not in those criteria.
People still compute the ratio in AKI because it sits on the basic metabolic panel. Treat it as one cheap clue. Urinary obstruction, interstitial nephritis, and glomerulonephritis will be missed if the ratio is the only tool. In hospital, plot both BUN and creatinine against time. Parallel rise with oliguria is more informative than a single ratio of 22.
Upper GI bleeding is a classic high-ratio story because digested blood is a protein load. The ratio does not locate the bleed. A BUN of 40 with creatinine 1.0 (ratio 40) plus black stool and a falling hemoglobin is a bleeding workup. The same ratio on a high-protein tube feed with a dry exam is a nutrition and volume problem. Urea mmol/L to BUN mg/dL uses 2.8 because urea molecular weight is 60 and BUN counts nitrogen mass (28/60 ≈ 0.467, and 1 mmol/L urea ≈ 2.8 mg/dL BUN). Reverse the conversion and a urea of 8 mmol/L is not a BUN of 8. Confirm the analyte name on every overseas printout.
Unit pitfalls on overseas reports
A European printout that lists urea 8.5 mmol/L and creatinine 88 µmol/L is not a ratio of 8.5/88.
Convert urea to BUN 8.5 × 2.8 ≈ 24 mg/dL and creatinine 88/88.4 ≈ 1.00 mg/dL. The ratio is about 24. Entering 8.5 and 88 in mixed units produces 0.1, which looks like a laboratory disaster and is only a unit error. Some panels report both urea and BUN. Use one analyte. Adding them or averaging them has no physiologic meaning.
Limitations and safety
The 2.8 conversion from urea mmol/L to BUN mg/dL is a commonly used factor (urea nitrogen is 28 g/mol; BUN counts nitrogen mass, not whole urea). Laboratories may use a slightly different factor. Prefer a panel that already reports BUN.
This page does not diagnose prerenal azotemia, GI bleeding, or ATN. It divides two laboratory numbers. A ratio of 25 with creatinine 0.8 and a ratio of 25 with creatinine 3.5 are not the same problem. Read the absolute creatinine, the potassium, the urine output, and the rate of change. The ratio is a footnote.
Some records auto-compute the ratio and flag “high.” Those flags recycle the teaching cutoff of 20. They do not know about GI bleeding, steroids, or a high-protein diet. Turn the flag into a bedside question rather than an order set. After a large gastrointestinal bleed, BUN can stay high for a day while creatinine is still near baseline. After a saline bolus, both may fall. Recalculating the ratio every few hours during resuscitation is less useful than watching urine output and potassium. In pregnancy, BUN and creatinine both fall with higher GFR. A “normal” nonpregnant ratio can hide a problem if you ignore the lower absolute creatinine typical of mid pregnancy. Obstetric laboratories use pregnancy-specific ranges. Chest pain, black stools, fainting, no urine, or severe weakness need emergency evaluation rather than ratio interpretation.
How it works
Ratio = BUN (mg/dL) / creatinine (mg/dL). Urea in mmol/L is converted with BUN mg/dL ≈ urea mmol/L × 2.8. Creatinine in µmol/L is divided by 88.4.
Frequently asked questions
- What is a usual adult ratio?Many laboratories list about 10 to 20 when both analytes are in mg/dL. The range is a laboratory comment, not a disease cutoff.
- Does a high ratio prove dehydration?No. Gastrointestinal bleeding, corticosteroids, high protein intake, and reduced kidney perfusion can all raise BUN more than creatinine. Low ratios occur with low protein intake, pregnancy, and some intrinsic kidney injury.
- Should I convert urea to BUN first?If the lab reports urea in mmol/L, choose that unit. Do not divide urea mmol/L by creatinine mg/dL without converting.