Blood Urea Nitrogen (BUN) Test

Overview

The blood urea nitrogen (BUN) test measures how much urea nitrogen is in a person’s bloodstream. Urea nitrogen is a waste product that forms when the liver breaks down protein and converts ammonia into urea.

The blood carries this urea to the kidneys, which filter it—along with other waste materials—from the bloodstream. These waste products are then removed from the body through urine.

When BUN levels are higher than normal, it may indicate that the kidneys are not functioning properly. Below is a simple outline of this process:

Step What Happens
1. Liver Converts ammonia into urea following the breakdown of proteins.
2. Blood Carries urea from the liver to the kidneys.
3. Kidneys Filter out urea and other waste products.
4. Urine Waste leaves the body through urine.

Reasons for Ordering the Test

Doctors order a blood urea nitrogen (BUN) test for several medical situations. This test helps check how well the kidneys are working and can show if there may be problems with kidney function or kidney disease.

People with conditions like diabetes, high blood pressure, or chronic kidney disease often get this test to monitor changes in their kidney health. Doctors may also request a BUN test for those with symptoms of kidney failure, such as swelling, fatigue, or changes in urination frequency.

This test helps monitor patients with heart problems, especially congestive heart failure, since these issues sometimes affect the kidneys. Doctors also use the BUN test to assess how well dialysis is working for someone with kidney failure or renal insufficiency.

Other reasons for using this test include:

  • Monitoring people after a kidney injury or acute kidney injury.
  • Looking for signs of dehydration, hypovolemia, or urinary tract obstruction.
  • Checking for problems from liver disease, malnutrition, or gastrointestinal bleeding.
  • Helping diagnose other kidney-related problems such as glomerulonephritis, pyelonephritis, and acute tubular necrosis.

Doctors usually run the BUN test alongside a creatinine test and calculate the glomerular filtration rate (GFR) for a more complete picture of kidney health.

Steps to Get Ready

For most Blood Urea Nitrogen (BUN) tests, patients can eat and drink as they normally would. If the test accompanies other blood tests, sometimes fasting is needed. People on dialysis should consult their doctor for special instructions.

What Happens During the Procedure

During a blood test to check kidney function, a healthcare worker places a needle in a vein in the arm to collect a blood sample. The process is usually quick and causes only mild discomfort. The laboratory then analyzes the sample.

Key Points

  • No special preparation needed.
  • Normal activities can be resumed right after.
  • Minimal discomfort for most people.

Test Outcomes

Blood urea nitrogen (BUN) test results usually appear in milligrams per deciliter (mg/dL). A standard BUN level for most adults typically falls between 6 and 24 mg/dL.

When measured in millimoles per liter (mmol/L), the normal range is usually from 2.1 to 8.5 mmol/L. However, normal limits may change slightly depending on the laboratory’s standards and the patient’s age.

Age and BUN

  • Infants tend to have lower BUN values than adults.
  • In children, the normal range varies by age group.
  • Older adults may have slightly higher BUN levels, even without obvious health problems.

High BUN results often suggest that the kidneys may not be working properly. At the same time, various factors unrelated to kidney function can also lead to increased BUN, including:

  • Lack of enough water in the body (dehydration)
  • Blockage in the urinary tract
  • Heart problems such as congestive heart failure or a recent heart attack
  • Bleeding inside the digestive system
  • Shock or traumatic injury
  • Major burns
  • Certain medications (such as some antibiotics)
  • Diets high in protein

Related Kidney Markers

Test Name Normal Range What It Shows
Serum Creatinine (Scr) 0.74–1.35 mg/dL (men), 0.59–1.04 mg/dL (women) Kidney function
Albumin 3.5–5.0 g/dL Protein status; low levels may suggest damaged kidneys
Glomerular Filtration Rate (GFR) 90–120 mL/min/1.73 m² Overall kidney function; lower rates indicate reduced function
Blood Creatinine Usually same as serum creatinine Kidney efficiency in filtering waste

High BUN along with raised creatinine or low GFR typically supports evidence of kidney damage or decreased function. If results fall outside the usual range, consult a healthcare provider for further review and next steps. They may suggest additional tests or lifestyle changes based on the results.


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