BUN stands for blood urea nitrogen. Creatinine is a natural product of muscle breakdown that occurs at a low level in the body. Both BUN and creatinine are filtered by the kidney and excreted in urine. For this reason, BUN and creatinine are used together to measure kidney function.
If kidney function begins to decline, BUN and creatinine rise. A normal creatinine depends on muscle mass and age. In general, a normal creatine is 0.5 to 1.2 milligrams per deciliter (mg/dL). A normal BUN is 7 to 20 mg/dL. Remember that different laboratories use different testing equipment, and may have slightly different ranges for normal.
A small, temporary increase in either BUN or creatinine can occur during illness or dehydration; the numbers usually return to normal during recovery.
In cases of dehydration, BUN may increase in a higher proportion than the creatinine. A normal BUN-to-creatinine ratio is between 10 to 1 and 20 to 1. (For example, a BUN of 14 and a creatinine of 1.2 would be a ratio of 12.)
In cases of dehydration, the BUN-to-creatinine ratio may be higher than 20 to 1, as the kidney responds by making the urine more concentrated. Other conditions that cause BUN-to-creatinine ratio to rise are kidney disease, kidney stones, or bleeding in the intestines. Low BUN-to-creatinine can occur in pregnancy, liver disease, or a diet low in protein.
The BUN-to-creatinine ratio is not a precise or reliable test, though. It may often be somewhat abnormal in healthy people. More important is the creatinine level itself; if it’s elevated, the kidneys are not working properly, and the reason should be sought out.
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