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Why would a doctor order a BUN-to-creatinine test?

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Doctors use a blood urea nitrogen-to-creatinine test to ascertain a patient's kidney function better than either test alone, according to MedicineNet.com. Levels of both substances may indicate dehydration, bacteremia, prostate cancer or kidney disease.

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A BUN-to-creatinine ratio helps doctors determine if a patient suffers from acute or chronic kidney disease, congestive heart failure or a urinary tract obstruction, says Quest Diagnostics. The BUN-to-creatinine ratio normally ranges from 6 to 22. Ranges of BUN and creatinine differ by age and sex, generally increasing as patients get older.

Creatinine levels in the blood rise abnormally when the kidneys fail to remove enough of the waste chemical into urine, says MedicineNet.com. Doctors compare how much creatinine is in the blood versus how much gets flushed out with the kidneys over a 24-hour period. Normal results are based on an individual's height, weight and age.

A BUN test measures how much nitrogen is in the blood produced by the waste product urea, according to WebMD. Urea gets made after the liver helps break down protein and gets flushed out of the body with the kidneys. BUN levels rise when kidneys do not properly remove the substance from the body. Higher-than-normal BUN amounts may indicate heart failure, dehydration or a high-protein diet. Lower BUN levels can indicate liver disease.

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