Blood Urea Nitrogen (BUN) Test

General Description

The blood urea nitrogen (BUN) test is performed to determine whether the kidneys are functioning properly. These organs are responsible for filtering the blood and eliminating waste products. In this process, they remove the urea produced by the liver (present in the blood as nitrogen) from protein metabolism. Elevated urea levels indicate potential renal impairment and may also suggest gastrointestinal bleeding, dehydration, or altered protein catabolism.

When is it indicated?

The BUN test is part of examinations to evaluate kidney function. These tests are usually performed in patients with symptoms of renal insufficiency, those on dialysis, or patients with pyelonephritis or urinary tract infection.

When dehydration is suspected, the blood urea nitrogen test is often combined with a creatinine test. A high BUN-to-creatinine ratio—that is, a higher amount of blood urea nitrogen compared to creatinine (a muscle metabolism waste product)—is often due to dehydration, which can cause impaired kidney function or urinary flow obstruction.

This test is also used to monitor the progression of kidney disease or assess the response to treatment.

How is it performed?

The BUN test is part of standard blood work. The first step involves blood collection. The patient sits with their arm extended while the clinician applies a tourniquet above the elbow. A needle is then inserted into a vein in the inner elbow, and the blood is collected in a sterile tube after removing the tourniquet.

A sterile tube with a red cap (no additives) or a yellow cap (containing a separating gel that promotes coagulation and separates serum from plasma after centrifugation) can be used. Tubes with fluoride or ammonium anticoagulants should be avoided, as they interfere with the process.

In the laboratory, two methods can be used:

• Manual technique: A small amount of blood is mixed in a cuvette at 37°C with reactive substances (NADH, urease, glutamate dehydrogenase, oxoglutarate, and non-reactive stabilizers). Absorbances are then read at 30 and 90 seconds.
• Automated technique: Technological devices allow biochemical analysis of the blood by simply inserting the sample and setting specific parameters. The clinician must calibrate the device to obtain reliable results.

Risks

The BUN test does not pose a health risk to the patient.

What to Expect from a Blood Urea Nitrogen Test

As blood is normally drawn from a vein in the arm, patients are advised to wear short sleeves or easily removable clothing. A brief pinch may be felt when the needle is inserted, which subsides quickly. To minimize the risk of bruising, pressure should be applied to the puncture site for several minutes.

Patients with a history of fainting during blood draws should inform the clinician beforehand to allow for supine positioning during and immediately after the procedure.

Normal blood urea levels (10–20 mg/dL in adults and 5–18 mg/dL in children) serve only as a reference and should be interpreted alongside other test results by a specialist. Elevated results may indicate kidney disease, dehydration, or heart failure impairing blood flow to the kidneys. Lower-than-normal results can be due to malnutrition, very low protein intake, or excessive fluid consumption.

Results are generally available within one to two days after the laboratory receives the sample.

Specialties in which blood nitrogen analysis is requested

BUN testing can be requested in various specialties, including Nephrology, Urology, Cardiology, Emergency department, Internal medicine, Pediatrics and Family and community medicine. The test is conducted by the Clinical analyses.