Blood urea nitrogen, also called BUN, is a biochemical test that assesses the amount of nitrogen in the blood. Specifically, it assesses the nitrogen that is produced from urea. Urea is a byproduct of protein breakdown in the liver and is cleared out of the body through the kidney.
How the Test is Performed
A blood urea nitrogen is determined by performing a simple blood test. The blood sample is obtained from a large vein in the arm, ideally the cephalic vein, though it may be from drawn from any other available vein if required. The injection site is cleaned with antiseptic solution and a tourniquet is applied to the upper arm to make the veins prominent. A sterile needle is inserted into the vein and the required amount of blood is aspirated into a syringe. Once the sample is drawn, it is sent across to the biochemistry laboratory for analysis. The injection site is covered with a bandage.
Before the test, patients are requested not to consume foods such as meats or other high protein foods for at least 24 hours. This prevents blood urea levels from returning as falsely elevated.
Patients who require a BUN assessment will need to stop certain medication before the test as they can affect the results. These drugs include (and are not limited to) steroids, antibiotics such as tetracycline and tobramycin and antifungal drugs such as amphotericin B.
Clinical Implications
Blood urea nitrogen values reflect the underlying function of the kidney. Normal values lie between 6 to 20 mg/dL (2.1 – 7.1 mmol/L), though they may vary between laboratories.
The commonest cause for high BUN is kidney disease. Kidney damage can be due to conditions such as high blood pressure and diabetes. Intrinsic diseases of the kidney such as glomerulonephritis and tubular necrosis can also cause elevated BUN levels.
High Levels of BUN
Individuals who consume a high protein diet can also have a high BUN.
Elevated levels of BUN can be seen in clinical conditions such as congestive cardiac failure (weakness of the heart muscle), dehydration and gastrointestinal bleeding. Low blood pressure due to blood loss (or dehydration) is also a recognised cause. Blood loss can also cause circulatory shock, which is characterized by high BUN levels. Heart attacks can also be a cause of high BUN.
There are certain cases where the BUN is below the normal range. These include liver failure, poor nourishment and excessive hydration (seen when patients are given too much intravenous fluids or drink too much water). Low BUN is also seen in after the first trimester of pregnancy.
Conclusion
Blood urea nitrogen is an extremely useful test in a number of different clinical conditions. Elevated values warrant immediate treatment to bring levels back to normal.