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BUN (Blood Urea Nitrogen)
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

BUN (Blood Urea Nitrogen)

The BUN test measures the amount of urea nitrogen in your blood to assess how well your kidneys are functioning.

Overview

Urea nitrogen is a waste product created when your body breaks down proteins. It is filtered from the blood by the kidneys and excreted in urine. Elevated BUN levels can indicate that the kidneys are not working properly, which may lead to complications like high blood pressure, anemia, or heart disease.

Why do I need a BUN test?

The test is often part of a comprehensive metabolic panel (CMP). It is used for routine checkups, monitoring kidney disease, or screening individuals at risk due to factors such as diabetes, high blood pressure, heart disease, obesity, age over 60, or a family history of kidney issues. It may also be ordered if you experience symptoms of kidney disease, such as fatigue, swelling, muscle cramps, or changes in urination frequency.

What happens during a BUN test?

A healthcare professional will collect a blood sample from a vein in your arm using a small needle. The process typically takes less than five minutes.

Preparation and Results

Usually, no special preparation is required, though you may need to fast if other tests are being performed simultaneously. High BUN levels can be caused by kidney dysfunction, but also by dehydration, burns, certain medications, a high-protein diet, or a recent heart attack. Low levels may be linked to malnutrition or liver disease. Because BUN is only one measure of kidney function, your provider may also order tests for creatinine or GFR (Glomerular Filtration Rate) to get a complete picture of your kidney health.

Frequently Asked Questions

In adults, a normal BUN range is roughly 7‑20 mg/dL (2.5‑7.1 mmol/L). Values consistently above 20 mg/dL may signal reduced kidney clearance, dehydration, or high protein catabolism. Persistent elevations >30 mg/dL usually prompt further evaluation with creatinine, eGFR, and possibly imaging.
Dehydration concentrates blood urea, often raising BUN by 5‑10 mg/dL even if kidney function is normal. To obtain an accurate baseline, drink adequate fluids (about 2 L of water) the day before the draw, unless your doctor has instructed fasting for other tests.
high BUN with normal creatinine often points to non‑renal factors such as dehydration, a high‑protein diet, gastrointestinal bleeding, or corticosteroid use. It can also occur after a recent heart attack or severe burns, where protein breakdown is increased. Your clinician will review medications, diet, and recent illnesses to pinpoint the cause.
Generally, you do not need to alter your diet unless you are on a very high‑protein regimen (e.g., bodybuilding supplements) that could skew results. If you are taking diuretics, ACE inhibitors, or certain antibiotics (e.g., trimethoprim‑sulfamethoxazole), inform your provider, as they can affect BUN. No fasting is required unless other labs are ordered, but follow any specific instructions given by the lab.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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