Blood urea nitrogen tests are diagnostic tests performed by doctors to determine whether or not patients' livers and kidneys work properly, as stated by WebMD. These tests measure the amount of urea, a protein deriving from waste produces, in the bloodstream. Livers naturally produce certain amounts of urea, but excessively high levels of urea in the blood indicate kidneys cannot effectively remove excess urea, in turn demonstrating impaired performance.
Doctors use blood nitrogen tests, or BUN tests, alone or in conjunction with other diagnostic tests, such as blood creatine tests. Using both tests provides further detail into kidney function, and helps physicians identify causes of kidney problems. Abnormal levels of urea in the blood may stem from other causes besides kidney and liver malfunction, as noted by WebMD. Following diets high in protein may elevate urea levels, as can heart failure and dehydration. Abnormally low levels of urea may result from liver diseases and damage or pregnancy.
In addition to helping diagnose problems, physicians use BUN tests to monitor certain medical conditions. Having repeat tests shows whether patients' conditions worsen, improve or stay the same. In preparation for a BUN test, experts recommend avoiding a high-protein meal at least 24 hours before the test, as protein naturally elevates urea levels. On test day, practitioners perform the test by drawing blood, typically from the arm or hand.