A blood urea nitrogen, or BUN, and creatinine test are usually used in conjunction to measure kidney function, help diagnose kidney disease or monitor kidney status in those who have already been diagnosed with kidney disease. BUN and creatinine tests are often ordered when various signs and symptoms indicate that a kidney disorder might be present.
Normal BUN
When your body metabolizes protein, your liver produces nitrogen. Nitrogen combines with other molecules in your liver to form a waste product called urea. Urea travels through your bloodstream until it reaches your kidneys, where it is filtered out of the blood and deposited into your urine. Normally, the blood and urine both contain small amounts of urea because protein is constantly being metabolized in your body. Normal BUN values range from 7 to 20 mg/dL.
Abnormal BUN
High BUN values can indicate kidney disease, congestive heart failure, gastrointestinal bleeding, shock, kidney failure, hypovolemia, a heart attack or a urinary tract obstruction. Low BUN values may occur as a result of liver failure, malnutrition or over-hydration, which is a condition in which too much fluid accumulates in the body.
Normal Creatinine
Your muscles produce a waste product called creatinine when they break down a compound called creatine, which helps contract your muscles. Most of the creatinine in your body is removed by your kidneys. Because of this, the levels of creatinine in your blood can indicate whether your kidneys are functioning properly. Normal creatinine values range from 0.8 to 1.4 mg/dL. Normal creatinine values for females are usually lower because females usually have a lower muscle mass.
Abnormal Creatinine
If your creatinine values are higher than normal, it can indicate kidney failure, glomerulonephritis, dehydration, urinary tract obstruction, diabetic nephropathy or reduced blood flow to the kidneys due to congestive heart failure or shock. Low creatinine values may occur as a result of muscular dystrophy or myasthenia gravis.
BUN/Creatinine Ratio
When a blood sample is tested, a doctor may evaluate the amount of BUN in your blood compared to the amount of creatinine, or the BUN/creatinine ratio. This ratio can be used to determine what type of kidney disease is causing abnormal levels of BUN or creatinine. Normal BUN/creatinine ratios fall between 10 to 1 and 20 to 1. An increased ratio can indicate congestive heart failure or dehydration. A decreased ratio can indicate gastrointestinal bleeding, liver disease or malnutrition.
