BUN blood test: what high or low BUN means

What BUN measures

When you digest protein, your liver turns the leftover nitrogen into urea. Urea travels in the blood to the kidneys, which filter it into urine. BUN is the amount of that urea nitrogen in your blood, reported in milligrams per deciliter (mg/dL).

So BUN depends on three things: how much protein you take in and break down, how hydrated you are, and how well your kidneys filter. A change in any of them moves the number.

What a high BUN can mean

• Dehydration. Not drinking enough, sweating a lot, vomiting or diarrhea concentrates the blood and raises BUN, often more than creatinine.
• A high-protein diet. More protein means more urea. Eating a lot of protein can push BUN above the lab range even with healthy kidneys.
• Reduced kidney function. If the kidneys filter less, both BUN and creatinine tend to rise, and eGFR falls.
• Bleeding in the stomach or intestines. Digested blood is a protein load and can raise BUN.
• Medications, such as the antibiotic tetracycline, the anti-seizure drug carbamazepine and methotrexate.
• Other causes: a blockage in the urinary tract, a recent heart attack, burns, major stress and older age.

What a low BUN can mean

A low BUN is usually not a concern. It most often reflects low protein intake, a small body size or drinking a lot of fluid. Less often it can go with malnutrition or liver disease, since the liver makes urea.

BUN, creatinine and eGFR

Creatinine is a waste product from normal muscle use, also cleared by the kidneys. Labs use it to calculate eGFR, an estimate of how well your kidneys filter. A GFR of 60 or more is in the normal range, and below 60 can mean kidney disease. Gabriel's optimal eGFR is 90 to 160, and creatinine ranges differ for men and women.

Many labs also report the BUN to creatinine ratio. It is a clue, not a diagnosis:

What to do with an abnormal result

1. Check creatinine and eGFR first. A high BUN with normal creatinine and eGFR points away from a kidney problem.
2. Think about the day before the draw. Little water, a big protein meal, a hard sweaty workout or a stomach bug can all raise BUN.
3. Ask about a urine albumin test (UACR) if creatinine or eGFR is also off. Damaged kidneys let albumin, a protein, leak into the urine.
4. Retest well hydrated. Drink normally the day before and keep protein at your usual level so the recheck reflects your baseline.

See a clinician promptly if an abnormal BUN comes with very little urine, swelling in your legs or face, confusion, black or bloody stools, vomiting blood, or signs of severe dehydration such as dizziness on standing and a racing heart.