BUN and creatinine are both waste products the kidneys clear. Dividing one by the other helps separate dehydration from a problem in the kidneys themselves.
The BUN/creatinine ratio is BUN divided by creatinine, both in mg/dL. A ratio between 10 and 20 is the usual range. Above 20 most often points to dehydration or reduced blood flow to the kidneys. Below 10 can reflect a low-protein diet, liver disease or kidney injury. Read it alongside the two values, not on its own.
The ratio narrows down where a kidney-marker change comes from. It does not replace the two values. A ratio outside 10 to 20 with normal BUN and creatinine is common and often means little.
| Ratio | What it usually suggests | Common causes |
|---|---|---|
| Above 20 | BUN is high relative to creatinine. The problem is often before the kidneys (called pre-renal). | Dehydration, reduced blood flow to the kidneys (for example heart failure), bleeding in the upper gut, a high-protein diet, corticosteroids |
| 10 to 20 | The usual range. If BUN and creatinine are both raised while the ratio stays here, the kidneys themselves are the more likely source. | Normal kidney function, or kidney disease raising both values together |
| Below 10 | BUN is low relative to creatinine. | Low protein intake, liver disease (the liver makes urea), muscle breakdown (rhabdomyolysis), intrinsic kidney injury, overhydration |
Creatinine is filtered by the kidneys and barely reabsorbed. Urea is partly reabsorbed, and when the body is short of fluid the kidneys hold on to more water and more urea with it. BUN climbs faster than creatinine, and the ratio rises above 20. Once fluid is restored, the ratio usually settles back. Our guide to dehydration on blood tests covers the other markers that shift at the same time.
When the kidneys themselves are damaged, both waste products build up together, so the ratio tends to stay between 10 and 20 even as the individual values rise. That is why a normal ratio with a high creatinine deserves more attention than a high ratio with normal values. Your eGFR, which our eGFR calculator works out from creatinine, is the better measure of kidney function itself.
Muscle mass matters too. Creatinine comes from muscle, so people with less muscle, including many older adults, run a lower creatinine. The ratio then drifts above 20 with no kidney or fluid problem at all.
The 10 to 20 range assumes both values are in mg/dL. Most UK and European reports print urea in mmol/L and creatinine in µmol/L, so dividing the printed numbers gives a figure on a different scale.
BUN mg/dL = urea mmol/L × 2.8
creatinine mg/dL = creatinine µmol/L ÷ 88.4
The factor 2.8 applies because BUN counts only the nitrogen in urea: each urea molecule carries two nitrogen atoms, weighing 28 in total. If your report prints whole urea in mg/dL instead, as some labs do, that figure is about 2.14 times the BUN. Divide by 2.14 before using it here.
Some UK reports also print a urea:creatinine ratio. It is calculated from mmol/L values and sits on a different scale, so don't compare it with the 10 to 20 range on this page. Use the calculator above with the two individual values instead.
References & Guidelines
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