What is a nitrogen test and should I ask my doctor for one?

What Is a Nitrogen Test?
A nitrogen test—most often called a blood urea nitrogen (BUN) test—measures the amount of nitrogen in your blood in the form of urea. Urea is a waste product produced by your liver when it breaks down proteins. Your kidneys filter urea out of your blood and send it out of your body through urine. By checking blood urea nitrogen levels, doctors can learn about how well your kidneys and liver are working, how your body is managing protein, and whether you’re properly hydrated.

Why Doctors Order a Nitrogen Test
A nitrogen (BUN) test is a common blood test that can provide clues about several aspects of your health:

• Kidney function. Because your kidneys clear urea from your blood, a high BUN can signal that they’re not filtering properly.
• Liver function. The liver makes urea as part of the urea cycle. Significant liver damage or cirrhosis can lower BUN levels.
• Hydration status. Dehydration concentrates your blood, raising BUN. Overhydration can dilute it.
• Protein intake and breakdown. Eating a very high-protein diet or experiencing rapid muscle breakdown can increase BUN.

Common Reasons to Check BUN
• Routine health exams or hospital panels
• Monitoring known kidney disease (e.g., chronic kidney disease)
• Evaluating acute kidney injury (sudden kidney damage)
• Assessing liver disease or cirrhosis severity
• Checking hydration status in critically ill patients
• Tracking progress in malnourished or post-surgical patients

Understanding Your Results
Normal BUN levels typically range from about 7 to 20 mg/dL (milligrams per deciliter), though exact ranges can vary slightly by lab. Results outside this range may mean:

High BUN (above normal)

• Kidney problems: reduced filtration in acute or chronic kidney disease
• Dehydration: less fluid means more concentrated blood urea
• High protein intake or rapid tissue breakdown (e.g., after a severe injury)
• Certain medications (e.g., corticosteroids, some antibiotics)

Low BUN (below normal)

• Severe liver disease or cirrhosis, where urea synthesis is impaired (Messing et al., 1995)
• Malnutrition or low protein diet
• Overhydration or fluid overload
• Pregnancy (due to expanded blood volume)

What a High or Low BUN Means for You
A single BUN result is just one piece of the puzzle. Your doctor will interpret it alongside other lab values (like creatinine), your medical history, physical exam, and, if relevant, symptoms such as:

• Swelling in legs or abdomen
• Changes in appetite or digestion
• Unusual fatigue or confusion
• Altered mental status (in liver failure, ammonia can build up and affect the brain—Butterworth, 2011)

Nitrogen Testing in Liver Disease
In patients with cirrhosis, the liver’s ability to convert ammonia to urea is reduced. This can lower BUN levels even as ammonia levels rise. Elevated ammonia contributes to hepatic encephalopathy, a serious brain-function problem (Butterworth, 2011). The European Association for the Study of the Liver (EASL, 2018) recommends monitoring ammonia and urea cycle markers in patients with decompensated cirrhosis to guide treatment and predict prognosis.

Should You Ask Your Doctor for a Nitrogen (BUN) Test?
You might consider discussing a nitrogen test with your doctor if you have:

• Known kidney disease or risk factors (high blood pressure, diabetes)
• Symptoms of kidney problems (changes in urination, swelling)
• Known liver disease or risk factors (heavy alcohol use, hepatitis)
• Symptoms of liver dysfunction (jaundice, confusion)
• Unexplained fatigue, loss of appetite or nausea
• Recent major surgery, injury or high-protein dieting
• Ongoing use of medications that can affect kidney or liver function

Preparing for the Test
• No special diet is usually required, but let your doctor know about high-protein meals or supplements.
• Tell your doctor about all medications and supplements, as some can alter BUN levels.
• You may be asked to fast (avoid food and drink) for a few hours before the blood draw.
• Drink enough water unless your doctor advises otherwise.

What to Expect
• A healthcare provider draws a small sample of blood, usually from a vein in your arm.
• You may feel a brief pinch or sting.
• The sample is sent to a lab; results typically return in a few hours to a couple of days.
• Your doctor will review the numbers with you and decide if further tests or treatment are needed.

When to Seek Medical Help
High or low BUN itself isn’t a diagnosis but a clue. If you experience any of the following, seek prompt medical attention:

• Severe confusion or altered mental status
• Sudden or severe swelling, especially in the abdomen (ascites)
• Difficulty breathing, chest pain or rapid heart rate
• Little to no urine output
• Signs of severe dehydration (dizziness, extreme thirst)

You might also consider doing a free, online “symptom check for Heart Failure” to explore whether your symptoms could fit another serious condition.

Key Takeaways

• A nitrogen (BUN) test measures urea nitrogen in blood and helps assess kidney and liver function, protein metabolism, and hydration.
• High BUN can mean kidney issues, dehydration or high protein breakdown. Low BUN may point to liver dysfunction, malnutrition or overhydration.
• Patients with liver cirrhosis can have low BUN but high ammonia, raising risk of brain dysfunction (hepatic encephalopathy).
• Talk to your doctor if you have kidney or liver disease risk factors, unexplained symptoms, or if you’re on medications that affect these organs.
• The test is a simple blood draw; your doctor will interpret it together with other information.

If you’re concerned about any serious symptoms or test results, speak to a doctor promptly. Only a healthcare professional can determine whether you need a nitrogen test, what the results mean for you, and what steps to take next.