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Kidney Diseases

Bun/creatinine ratio

03/03/2008

Question:

My urea nitrogen # was 22 and my creatinine was 0.80 - both normal. The bun/creatinine ratio is 27.5 (high). How can two normals equal a "high" ratio. Thanks.

Answer:

BUN (blood urea nitrogen) and creatinine are waste products that are produced by normal metabolic processes in the body and by the breakdown of foods, especially those high in protein. The normal value for BUN is approximately 10-20, and for creatinine, 0.5-1.2. The ratio between the two is usually between 10:1 and 20:1. However, if BUN is at the high end of normal and/or the creatinine is at the low end of normal, the ratio can actually be high.

Here are some possible explanations:

1) small body size and/or pregnancy: this could make the creatinine level be on the low side.

2) Mild dehydration: after an overnight fast, your body can become a little water-depleted, which could cause the BUN to be on the high side.

3) High protein diet: even a high-protein meal on the day before the test may increase the BUN without affecting creatinine, so that the ratio may become elevated.

4) Heart failure or liver cirrhosis: both of these conditions can cause a high BUN/creat ratio in the absence of kidney problems.

5) GI bleeding: if there is a bleeding ulcer or bleeding of any type anywhere in the stomach or intestines, the BUN will rise disproportionately from the creatinine.

6) Steroids: in people taking high doses of steroid medications such as prednisone (for instance, for asthma), the BUN can be elevated, causing a high calculated BUN/creatinine ratio.

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Response by:

Mildred   Lam, MD Mildred Lam, MD
Associate Professor of Medicine
School of Medicine
Case Western Reserve University