Blood urea nitrogen is low: what a low BUN means

Your liver turns the nitrogen left over from protein breakdown into urea, and your kidneys clear that urea into urine. A BUN result therefore reads two things at once: how much urea is being made, and how much water it is dissolved in. A low BUN means less urea made or the same urea spread through more water, so a low result says very little about your kidneys and a lot about your diet, your hydration and, occasionally, your liver.

For how BUN behaves across its whole range, see the main blood urea nitrogen page. For specific readings, the BUN of 5 mg/dL and BUN of 6 mg/dL pages walk through each value.

Why BUN goes low: less urea made, or more water

Urea is made in the liver from ammonia released when amino acids are broken down, then filtered freely by the kidneys and partly reabsorbed, so BUN depends on production and on body water as well as on kidney filtration ². Urea accounts for more than three-quarters of the non-protein nitrogen the body excretes, which is why BUN tracks protein intake so closely ².

BUN is a supply-and-dilution number as much as a filtration number: kidney failure retains urea and raises BUN, so a low BUN points to the liver's output or to body water, not to the kidneys ². The BUN high page puts the other side plainly: "Kidney trouble is likely only when creatinine is also high or eGFR (your estimated kidney filtration) is low." On the low side, reduced kidney filtration is not what the number points to.

The common, harmless reasons

Most low BUN results are found by chance, because BUN sits on every basic and comprehensive metabolic panel ¹. The usual explanations, roughly in order of how often they turn up:

• Eating little protein. The adult recommended intake is 0.8 g per kg of body weight per day, about 56 g for a 70 kg adult ⁷. Habitual intake well under that, common on plant-heavy diets, restrictive diets or long fasts, cuts urea production. If a clinician has limited your protein for kidney or liver disease, a low BUN may be intended.
• Drinking a lot of fluid before the blood draw, which dilutes urea for a few hours.
• Pregnancy. Kidney filtration rises by about 40 to 50 percent and plasma volume (the liquid part of the blood) expands, so BUN typically falls, often to around 8 to 10 mg/dL and sometimes below the usual range ³.
• Being small, young or low in muscle. Children and slight adults make less urea.
• Lab noise. A result of 5 or 6 mg/dL sits close to the lower limit, and a repeat may land inside the range.

CalculatorProtein calculatorCheck whether your usual protein intake reaches 0.8 g per kg, the first thing to rule out with a low BUN.Work it out

The reasons worth checking

MedlinePlus gives the standard short list of what a low result can mean ¹:

Lower-than-normal level may be due to: liver failure, low protein diet, malnutrition, over-hydration.MedlinePlus Medical Encyclopedia, BUN blood test (US National Library of Medicine)

The less common causes each leave a second abnormal result on the panel, which is how they are told apart from a harmless low BUN:

• Advanced liver disease or cirrhosis. A badly damaged liver makes less urea, so BUN can be low even while ammonia is high; albumin is usually low and bilirubin high. BUN falls mainly when liver failure is advanced ², so earlier problems such as fatty liver rarely lower it.
• SIADH and other water excess. In a person with low sodium, a low urea supports SIADH over dehydration ⁴. In SIADH, as the low sodium page puts it, "ADH keeps running, so the kidneys keep water they should pass."
• Malnutrition or malabsorption: eating disorders, coeliac disease, frailty in older adults. Total protein and albumin tend to drift down too.
• Urea cycle disorders. High ammonia is the hallmark of these inherited enzyme defects, and urea is often low or low-normal alongside it. Milder forms can first appear in teenagers or adults, often set off by illness, a large protein load or drugs such as valproate ⁵.
• Planned low-protein diets. US kidney guidelines (KDOQI, 2020) suggest about 0.55 to 0.6 g/kg/day for stable people with non-diabetic chronic kidney disease not on dialysis, which lowers urea on purpose ⁶. A low BUN right after dialysis is also expected.

Read it with the rest of your panel

For the sodium side of that table, electrolytes explained covers the basics, and clotting tests explained covers the INR.

When the BUN creatinine ratio is low

The BUN to creatinine ratio divides BUN by creatinine (both in mg/dL), and a ratio below about 10 to 1 counts as low ². A low ratio can come from either number. A low BUN drives it down for all the reasons above: low protein intake, liver disease, SIADH and pregnancy. Right after dialysis the ratio also falls, because dialysis clears urea faster than creatinine. A high creatinine with a normal BUN does the same, which happens with large muscle mass, creatine supplements and muscle breakdown (rhabdomyolysis).

A strength athlete with BUN 12 mg/dL and creatinine 1.4 mg/dL has a ratio of about 9. That ratio reflects muscle mass and creatine use, and the kidneys may be fine, though a clinician may confirm with a cystatin C test, since a creatinine of 1.4 can also mean lower kidney filtration. The creatine page explains why creatine raises creatinine. Outside the US, where results come as urea in mmol/L and creatinine in µmol/L, convert to the US units before dividing, or the ratio will be meaningless.

CalculatorBUN to creatinine ratio calculatorEnter your BUN and creatinine to see your ratio, in US or SI units.Work it out

Does low BUN cause symptoms?

Low urea causes no symptoms. Urea is a waste product, and a low level is not known to cause harm by itself. When someone with a low BUN feels unwell, the symptoms come from the cause: swelling, easy bruising or jaundice with liver disease; headache, nausea and confusion with low sodium; weight loss and tiredness with malnutrition; drowsiness, vomiting and confusion with high ammonia.

What to do about a low BUN

1. 1Look at the rest of the panelIf sodium, albumin, total protein and liver enzymes are normal, a low BUN on its own is almost always benign.
2. 2Think about the day of the testA large drink of water, a long fast or a low-protein week before the draw can each pull BUN down. The guide on how to prepare for a blood test covers fluid and fasting.
3. 3Check your protein intakeCompare your usual intake with 0.8 g per kg per day ⁷. Eating enough protein is the only lifestyle change with a clear link to BUN; no supplement is shown to raise BUN, and there is no reason to try. If you have kidney disease, liver disease or a urea cycle disorder, or a clinician has set your protein intake, keep to that plan and do not raise protein on your own.
4. 4Repeat it if it stays isolatedAsk your clinician whether to repeat it in 2 to 4 weeks, after a normal day of eating and drinking. A second low result with everything else normal rarely needs more.
5. 5Ask about further tests if anything else is offLow sodium, low albumin, abnormal liver tests or symptoms change the picture, and a clinician will usually add urine sodium and urine osmolality (how concentrated the urine is), an INR (a clotting test) or an ammonia level.

Units: BUN in mg/dL vs urea in mmol/L

US labs report BUN in mg/dL; UK, Australian and many European labs report urea in mmol/L. Multiply BUN in mg/dL by 0.357 to get urea in mmol/L ². (A few labs report urea itself in mg/dL, which is BUN multiplied by 2.14.) The urea page covers the SI result, and the lab units guide explains the conversions in general.

For where BUN sits among the other kidney tests, see kidney function explained; for the panel it arrives on, see the metabolic panel explained.

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