Urea is a nitrogen-containing waste product formed in the liver from the breakdown of amino acids and ammonia. It is transported to the kidneys in the blood and excreted in urine. Because urea is both produced by the liver and excreted by the kidneys, blood urea levels reflect both protein metabolism and kidney filtration function.

Elevated urea (uraemia) can indicate reduced kidney function, increased protein catabolism (from a high-protein diet, starvation, or muscle breakdown), upper gastrointestinal bleeding (where digested blood acts as a protein load), or dehydration. Low urea can be seen in liver disease (reduced production), malnutrition, or low protein intake. Urea is routinely measured alongside creatinine as part of kidney function testing.