What is BUN on a blood test?
BUN stands for blood urea nitrogen. It’s one line on the metabolic panel most people get with routine bloodwork, and it’s usually there to give your doctor a look at how your kidneys are working.
The short version
- Urea nitrogen is a waste product that forms as your body breaks down protein. Your blood carries it to your kidneys, which remove it when you urinate.
- The test measures how much of that waste is still in your blood. Having a small amount is normal.
- It isn’t purely a kidney number. The National Library of Medicine lists dehydration, burns, certain medicines, a high-protein diet, and a recent heart attack alongside kidney disease as things that can raise it.
- That’s why it’s read with creatinine. Comparing the two — the BUN/creatinine ratio — helps your doctor narrow down the cause.
- Medicare: BUN almost always arrives inside a panel. A comprehensive metabolic panel is covered when there’s a medical reason behind the order. You pay $0. Medicare pays the lab $10.56.
Where the number comes from
When your body uses protein, it produces waste. Your liver turns that waste into urea, your blood carries it, and your kidneys filter it out. MedlinePlus puts it plainly: urea nitrogen “is a waste product that forms as your body breaks down proteins. It’s carried in your blood and then removed by your kidneys when you urinate.”
So a BUN result is a snapshot of how much of that waste is still circulating. If the kidneys aren’t clearing it as well as they should, more of it stays behind. The National Library of Medicine notes that a BUN test “can help uncover kidney problems early, when treatment can be more effective.”
You rarely order a BUN by itself. It’s normally one of the measurements inside a basic or comprehensive metabolic panel — MedlinePlus notes it “is often part of a series of tests called a comprehensive metabolic panel (CMP).”
The BUN/creatinine ratio, and why it exists
If you see “BUN/CREAT RATIO” or “B/CR” on your result, that isn’t a test anyone ordered. It’s a calculation the lab prints automatically from two numbers already on the panel. That’s why it can appear on your report with no explanation attached.
Creatinine is the other number in that pair. It’s also a waste product your kidneys remove, but it comes from somewhere different: MedlinePlus describes it as “a normal waste product in your body. It’s made when you use your muscles and some of the muscle tissue breaks down.”
That difference in origin is the whole point. BUN moves with things that have nothing to do with your kidneys — how much protein you eat, how hydrated you are. Creatinine is steadier. So comparing them carries information neither one has alone. MedlinePlus states the purpose directly: creatinine levels measured on a panel “may be compared with the level of BUN (blood urea nitrogen) that’s measured in the same test. This can help find out the cause of a kidney problem.”
It’s a direction-narrowing number, not a diagnosis. Which direction it points is a reading your doctor makes, alongside your other results and your history.
What a high or low BUN can point to
These are the causes the National Library of Medicine lists. A result outside the range on your report does not by itself mean you have any of them.
| Result | Can point to |
|---|---|
| Higher than normal | Kidney disease. Also dehydration, burns, certain medicines, a diet high in protein, or a recent heart attack. |
| Lower than normal | A lack of protein in the diet, malnutrition, or liver disease. |
There’s no single “normal” number worth quoting here, and two things make that especially true of BUN. Each lab sets its own range and prints it beside your result. And MedlinePlus notes that BUN levels “can vary according to your sex. They also usually increase with age.” Compare your number to the range on your own report, not to one you find online. Why that range is not the whole story →
Before the test
No special preparation is needed for a BUN on its own. The catch is that it almost never travels alone: if your panel includes glucose or a lipid panel, those may require fasting, and you’ll be told to fast for the whole draw. More on fasting before blood work →
What Medicare pays
BUN has its own code, but you’ll rarely see it billed alone. It’s normally one measurement inside a panel, and the panel is what Medicare pays for. Both panels are covered when the order carries a medical reason — a condition being managed, a medication being monitored, or a symptom being worked up. Pure routine screening with no reason behind it is the exception.
| Test | Code | Medicare pays the lab | You pay if covered |
|---|---|---|---|
| Comprehensive metabolic panel | 80053 | $10.56 | $0 |
| Basic metabolic panel | 80048 | $8.46 | $0 |
| BUN alone | 84520 | $3.95 | $0 |
| Creatinine alone | 82565 | $5.12 | $0 |
The panel pages have the full coverage details and what each costs to buy directly if it won’t be covered. CMP cost and Medicare coverage →
Sources
- MedlinePlus (National Library of Medicine): BUN (Blood Urea Nitrogen)
- MedlinePlus (National Library of Medicine): Creatinine Test
- Medicare payment amounts: CMS Clinical Laboratory Fee Schedule, 2026 Q3
Checked September 23, 2026. This page explains what the test measures. It is not medical advice and cannot interpret your result. Your doctor can.
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