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Urinary Disorders

Bun and creatinine

06/25/2008

Question:

Recent labwork revealed an elevated BUN of 34 w/ a ref range of 5-26. Creatinine was 0.69 w/ ref range of 0.5-1.50. BUN/creatinin ratio was elevated at 49 w/ ref range of 8 to 27. This was a fasting sample (approx 12 hours w/o food or drink).... Never had abnormal BUN or the ratio before. No prior kidney problems. Could this be indicative of early kidney problems?...dehydration? Should I have this rechecked...if so, when? Also, I am scheduled for a CT scan this week secondary to some RUQ discomfort...should I refuse iodine contrast? 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.

Here are some possible explanations for a high BUN/creatinine ratio:

1) Mild dehydration: after an overnight fast, your body can become a little water-depleted, which could cause the BUN to be high (although yours is higher than I would expect from this alone).

2) 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.

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

4) 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.

5) 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.

6) Diuretics: if you are taking these for any reason, certain ones (especially hydrochlorothiazide) can increase the BUN slightly.

If at all possible, I would recommend that you have the CAT scan postponed until the BUN and creatinine can be repeated.  Before the next blood draw, you can eat and drink normally (not more than normal, just normal).  If the BUN is high again, some screening investigations should be done, such as a check for internal bleeding (by checking stools for blood), and a urinalysis.  These will help determine whether or not it is safe for you to undergo a CAT scan with contrast dye.

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Response by:

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