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Urea is a waste product from the metabolism of proteins excreted in the urine. The amount of urea in the blood is a measure of the renal function and, in some situations, dietary protein intake and liver function. Testing for blood creatinine and calculating the estimated glomerular filtration rate (eGFR) is usually preferred over urea testing to assess renal function.

High urea levels in the blood generally suggest bad renal function (often both kidneys), which can be acute or chronic (renal failure). High urea levels can also be caused by a condition that reduces the blood flow to the kidneys (congestive heart failure, shock, major stress, recent heart attack, urinary flow obstruction, dehydration). In rare cases, urea levels may suggest accelerated protein catabolism (intracellular digestion), a significant increase in the dietary protein intake or digestive bleeding (reabsorption of blood proteins in the intestine). Certain medications can also increase urea levels.

Lowered urea levels are most often of little clinical significance. However, they can reflect severe liver disease, malnutrition or too much water in the blood.

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