BUN, or blood urea nitrogen, is a test performed on a blood sample as an indirect test of your kidney function. Urea is a non-toxic waste product produced by your body to remove toxic ammonia produced by biochemical reactions. The BUN value from your blood test indicates how efficiently your kidneys are removing waste form your body.
Kidney Function
The primary function of the kidneys is the filtering of waste from the blood and removal via urine. Each day, your kidneys filter through a total of approximately 200 quarts of blood to remove up to 2 quarts of waste and excess water. When a doctor talks about kidney function, it is usually in reference to the percentage of waste removal capacity of your kidneys. Although a small reduction is not usually problematic, a large reduction could cause toxic metabolic waste to build up in your body.
The BUN Test
BUN is evaluated after taking a blood sample, usually as part of a metabolic panel testing a wide range of values. Your doctor may order the test if he suspects kidney issues, you have non-specific complaints or during a stay in the hospital. The BUN level reflects the waste clearance rate of your kidneys by measuring the backup of urea waste in the blood. Your level of BUN is compared against a reference range for a result of low, normal or high. The reference range will vary between lab to lab based on variability of reagents and procedure.
High BUN
A high BUN level indicates that urea has built up in your blood. Although this commonly means that your kidney function is impaired, it could also be due to excess production of urea in your body or decreased blood flow to the kidneys. Your doctor will use other tests, such as creatinine level, and more targeted tests of kidney function to determine whether you have kidney disease.
Low BUN
Low BUN levels are very uncommon and are not a cause for concern, but do not indicate kidney function above normal levels. Common causes of low BUN levels include liver disease, malnutrition, metabolic disorders or over-hydration that may been seed during intravenous administration. BUN tests are not used to diagnose any of the conditions in which low BUN levels are found.
