Early diagnosis of IgA nephropathy (IgAN) and other chronic kidney diseases can dramatically improve your quality of life. The sooner your doctor notices something amiss, the sooner you can manage the disease and slow its progression. Because kidney disease symptoms can take years to appear, healthcare providers rely on routine blood and urine tests for early detection. One of those tests is the blood urea nitrogen (BUN) test.

A BUN test usually isn’t enough to diagnose someone with kidney disease, but an abnormal result may prompt additional tests that do provide a clear diagnosis. If you have IgAN or another type of chronic kidney disease, your healthcare provider may order BUN tests every few months to monitor disease progression. This guide will help you understand what BUN is, what high and low BUN levels mean, and other kidney tests commonly run alongside BUN.

What Is BUN?

A BUN test measures the amount of nitrogen in your blood. That nitrogen is part of a larger compound called urea, a waste product that’s excreted through urine. Testing BUN gives healthcare providers important information about how well your kidneys are working.

To understand BUN’s role in identifying kidney problems, we need to start in the liver. When the liver breaks down protein for energy, it creates a nitrogen-based waste product called ammonia. Ammonia is highly toxic, so the body helps neutralize it by combining it with carbon, oxygen, and hydrogen. The final product, called urea, can safely enter the bloodstream. It travels to the kidneys, which clean the blood by filtering out waste products. Urea travels to the bladder and leaves the body during urination.

Normal BUN levels can vary based on age, sex, and health history, so talk to a healthcare professional about what your results mean.

Healthcare providers measure the amount of urea nitrogen in your blood to check how well your kidneys filter waste. Reduced kidney filtering is a common symptom of IgAN.

What To Expect With a BUN Test

You don’t need to fast for a BUN test. A small blood sample, about 0.5 to 1 milliliter (mL), is taken from a vein in your arm. That’s less than a quarter of a teaspoon. A trained healthcare professional called a phlebotomist — someone who draws blood for testing — may take a larger sample if your doctor has ordered additional blood tests.

The blood samples are then sent to a lab for analysis. It may take a few days to get your results and to hear from your doctor.

Interpreting BUN Test Results

Everyone has some urea in their blood, so your healthcare provider will look for BUN results outside of the normal range. Reference ranges vary by lab, but in general, a BUN higher than 22 to 26 milligrams per deciliter (mg/dL) is considered elevated in adults.

For children aged 4 to 17, BUN levels should typically be between 7 and 20 mg/dL. In infants and children up to age 3, BUN levels tend to be lower than 14 mg/dL, but the range can vary.

Normal BUN levels can vary based on age, sex, and health history, so talk to a healthcare professional about what your results mean.

What Does a High BUN Level Mean?

A high BUN level means that something isn’t working properly in your body, but it doesn’t automatically point to kidney disease like IgAN.

High BUN levels may also be caused by the following:

• Blocked urinary tract
• Dehydration
• High-protein diet
• Certain antibiotics or other medications
• Certain blood pressure medications, such as angiotensin-converting enzyme inhibitors (ACE Inhibitors) and angiotensin-receptor blockers (ARBs)
• Severe infection
• Recent heart attack
• Congestive heart failure
• Gastrointestinal bleeding
• Severe burns
• Shock
• Stress

What Does a Low BUN Level Mean?

A low BUN level is uncommon but may mean:

• Liver disease
• Too much water in your body (overhydration)
• Low-protein diet
• Smaller body size

Other Tests That Analyze Kidney Function

Because so many factors can raise or lower your BUN levels, your healthcare provider may order additional tests to pinpoint the problem.

Serum Creatinine

Serum creatinine (sCr) is a waste product like urea, and healthy kidneys should have no problem filtering it out of the bloodstream. By comparing the amount of BUN and creatinine in your blood (called the BUN-to-creatinine ratio), healthcare providers get a clearer picture of what’s happening inside your body. If you’re dehydrated or if your gastrointestinal tract is bleeding, BUN levels rise while creatinine levels stay the same. On the other hand, elevated levels of both creatinine and BUN may be a sign of a blocked urinary tract or kidney disease.

Estimated Glomerular Filtration Rate

Once your healthcare provider knows how much creatinine is in your blood, they can calculate your estimated glomerular filtration rate (eGFR). eGFR is a mathematical formula that considers your serum creatinine, age, and sex assigned at birth to determine if your kidneys are working correctly. An eGFR of 90 or above indicates healthy kidney function.

Cystatin C

It’s standard to calculate eGFR using serum creatinine levels, but sometimes additional testing is helpful. People who are older or who have significant muscle mass may require a test that measures the amount of cystatin C circulating in the bloodstream. Elevated levels of this protein can point to kidney problems, and cystatin C measurements can be used in place of serum creatinine in eGFR calculations.

Urinalysis

Your healthcare provider can also learn a lot about your kidneys by evaluating your urine. If they think you have kidney damage, they’ll be particularly interested in measuring the amount of creatinine and albumin (a protein) in your urine. The albumin-to-creatinine ratio (ACR) gives your healthcare provider a snapshot of your kidney health. The ideal ACR falls below 30, while an ACR of 30 to 300 indicates a moderate increase in albumin in the urine and an increased risk of kidney disease. An ACR above 300 is considered extremely high.

Your ACR is considered alongside your eGFR to figure out your overall risk for kidney damage. To rule out any temporary causes of high urine albumin, you may be asked to repeat the urinalysis in three months. Two high results in a row typically suggest kidney disease.

When To Talk to a Doctor

Many people get a BUN test as part of a routine comprehensive metabolic blood panel. Regular blood work helps your healthcare provider catch potential issues early. However, if you already have kidney damage or a kidney disease like IgAN, you may need more frequent BUN tests to monitor disease progression. Talk to your doctor about the best testing schedule for you.

Consider asking your doctor the following questions during your next visit:

• What do you consider a healthy BUN range for me?
• What was my latest BUN level? Is this considered slightly, moderately, or highly elevated?
• Is it possible to lower my BUN level? How?
• How can we rule out other possible causes of high BUN?
• Is a BUN test the most affordable/economical way to monitor my kidney health?
• Will I feel anything if my BUN level gets high or low?
• How often do you want to test my BUN level?

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