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Blood Test Tracker

Urea

Urea is the main waste product from breaking down protein. Your kidneys clear it from the blood.

AI-verified against cited sources. Awaiting expert reviewLOINC 3091-6

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What it is

Your liver makes urea when your body breaks down protein, and your kidneys filter it into urine. A urea test measures how much is in your blood. Labs outside the US usually report urea in mmol/L. US labs usually report BUN (blood urea nitrogen) instead. It is the same waste product counted a different way, so the numbers differ unless you use the right conversion.

Why it matters

Urea goes up when the kidneys clear less of it. It also goes up with dehydration, a high-protein diet or bleeding in the gut, so it isn't a stand-alone test of kidney function. Clinicians read it with creatinine and eGFR. A low urea is uncommon, and it is usually explained by diet, pregnancy or extra body water.

If it’s low

A low urea is uncommon. It usually reflects eating little protein, extra water in the body (including in pregnancy) or low muscle mass. Rarely, it can be a sign of reduced urea production from severe liver disease or an inherited urea cycle disorder.

Common causes

  • Low protein intake or poor nutrition
  • Pregnancy, when the body holds more water
  • Overhydration, including conditions that cause the body to hold on to water
  • Low muscle mass
  • Severe liver disease
  • Inherited urea cycle disorders (rare)

What can help

  • Tell your clinician about your diet, any recent illness and whether you are pregnant
  • Ask your clinician whether any other tests, such as liver tests, would be useful

If it’s high

A higher urea means more of it is staying in your blood. This often reflects being low on fluids or eating a lot of protein, not a kidney problem. But it can also be a sign that the kidneys are clearing less than usual. Many different things can cause this, and your clinician can put it in context with your creatinine and eGFR.

Common causes

  • Dehydration, such as low fluid intake, vomiting, diarrhea or heavy sweating
  • A high-protein diet or high muscle mass
  • Older age, since levels tend to rise gradually with age
  • Medicines such as corticosteroids or diuretics
  • Reduced blood flow to the kidneys, for example with heart failure or low blood pressure
  • Bleeding in the digestive tract
  • Reduced kidney function, either sudden or long-term
  • A blockage in the urinary tract
  • Serious illness, such as infection, burns or other causes of tissue breakdown

What can help

  • Ask your clinician whether your fluid intake is enough for you
  • Talk with your clinician about your protein intake and any supplements
  • Ask your clinician to look at urea together with creatinine and eGFR

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex and age. Choose women or men to see the one that applies.

Children and teens 2 ranges

See Urea by age and sex, with the U.S. survey numbers where they exist.

How common is your level?

NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.

How Urea varies with age in the U.S.Shaded: middle 90% and middle 50% of people. Line: the median. mg/dL. Point at any age, or focus the chart and use the arrow keys, to read both sexes in words.

Use the left and right arrow keys to read each age. Shift moves five years.

Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.

Explore Urea by age and sex: every percentile as a table, with a download.

What can affect your result

  • Age: levels rise slowly with age, and lab ranges are higher in older adults
  • Sex: slightly higher in men, partly because of more muscle mass
  • Protein intake and muscle mass
  • Hydration status
  • Pregnancy: usually lower because the body holds more water
  • Bleeding in the digestive tract
  • Medicines such as corticosteroids and diuretics
  • The unit your lab uses: urea in mmol/L, urea in mg/dL and BUN in mg/dL are three different numbers for the same waste product
  • Dehydration raises urea even when the kidneys work normally, and extra body water (overhydration) can lower it.
  • A high-protein meal or diet raises urea, and a very low-protein diet lowers it.
  • Bleeding in the digestive tract raises urea because the body breaks down the blood protein.
  • Corticosteroids and some other medicines raise urea by increasing protein breakdown or lowering fluid volume.
  • Very high blood ammonia (for example in severe liver disease) can interfere with some urease-based methods, but this is uncommon.

Can change within hours to days with fluid balance, protein intake and illness. With fluid loss or heart failure it tends to rise before creatinine does.

When to talk to a clinician

Talk with your clinician if your urea is above your lab's range, especially along with a high creatinine or a low eGFR. There is no standard urgent cutoff for urea alone. Contact a clinician soon if a high result comes with very dark urine or much less urine than usual, dizziness, vomiting that won't stop, black or bloody stools, or confusion. If your lab flagged the result as urgent, follow its instructions.

Questions you could ask

  • Why was urea included on my panel?
  • Should I repeat this test, and when?
  • Could dehydration, my diet or a medicine explain this result?
  • Should we look at my creatinine and eGFR together with this?
  • Could my diet, a pregnancy or my fluid balance explain a low value?
  • Is any other testing needed?

Questions people ask

Is urea the same as BUN?

They are the same waste product, counted two ways. BUN counts only the nitrogen part of urea. Urea counts the whole molecule. To convert BUN in mg/dL to urea in mg/dL, multiply by 2.14. If your report gives either one in mmol/L, the number is the same for both.

How do I convert urea from mmol/L to mg/dL?

Multiply mmol/L by 6.006. For example, 5.0 mmol/L is about 30 mg/dL. To go the other way, divide mg/dL by 6.006.

Can dehydration or a high-protein diet raise my urea?

Yes. Both are common, harmless reasons for a higher result. That is why clinicians look at urea alongside creatinine and eGFR before drawing any conclusion about the kidneys.

Is urea a good test of kidney function?

Not on its own. Creatinine and eGFR are preferred, because urea is affected by hydration, diet and many other things. Urea still adds useful context in some situations, such as dehydration or dialysis.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.1665 mmol/L
  • 1 mg/dL = 0.01 g/L

Open the Urea unit converter

Is a change real?

Urea naturally varies about 13.9% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 43% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical appraisal and meta-analysis of biological variation estimates for kidney related analytes (Jonker N et al., EFLM WG-BV).

For clinicians and researchers

Urea (CH4N2O, molecular weight 60.06) is the end product of the hepatic urea cycle and carries about 75% of the nitrogen the body excretes. It is filtered freely at the glomerulus, but part of the filtered load is reabsorbed in the tubules, so the amount excreted does not track GFR closely. At low urine flow, reabsorption rises, so plasma urea can climb with a normal GFR; this is why it is a weaker marker of GFR than creatinine. Production rises with protein intake, catabolism, corticosteroids and gastrointestinal bleeding, and falls with low protein intake, liver failure and urea cycle defects. Most labs measure urea with urease followed by glutamate dehydrogenase (NADH consumption at 340 nm), which is also the reference method. BUN reports only the nitrogen: urea (mg/dL) = BUN (mg/dL) × 2.14, and urea (mmol/L) = BUN (mg/dL) × 0.357, derived from the molecular weights of urea and nitrogen. A BUN given in mmol/L counts molecules of urea, so it equals urea in mmol/L. In well-hydrated people the urea:creatinine ratio (mmol/L divided by µmol/L) is about 0.04 to 0.06. Urea has a role in suspected prerenal failure, dialysis adequacy (Kt/V), prediction of acute pancreatitis severity (above 16.1 mmol/L within 48 hours in one scoring system) and the CURB-65 pneumonia score (above 7 mmol/L). The ACB analyte guide notes that routine urea has a thin evidence base and that creatinine with eGFR is preferred for kidney function.

LOINC 3091-6 (primary), 22664-7. Specimen: serum-or-plasma.

Sources

  1. Urea (serum, plasma): analyte guide, Association for Clinical Biochemistry and Laboratory Medicine (author W. Marshall) (2012)
  2. Urea (Clinical Biochemistry test catalogue), Oxford University Hospitals NHS Foundation Trust
  3. Urea (North West London Pathology test database), North West London Pathology
  4. Urea, serum (Newcastle Hospitals Laboratories test directory), Newcastle upon Tyne Hospitals NHS Foundation Trust
  5. Urea, serum (reference values by age and sex), Sonic Healthcare Australia (2018)
  6. Uréia, soro (Manual de Exames Fleury), Fleury Medicina e Saúde
  7. Urea reference ranges, DiaSys Diagnostic Systems GmbH
  8. Urea (PubChem compound 1176), NCBI PubChem

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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