UreaSerum urea · BUN
The waste product of protein breakdown, cleared by the kidneys.
Urea is what the liver makes when it disposes of the nitrogen left over from breaking down protein. It travels in the blood to the kidneys, which filter it into urine, so the level in blood reflects both how much protein is being processed and how well the kidneys are clearing it.
It matters as a second view on kidney function alongside creatinine, and as a marker that moves for non-kidney reasons: dehydration concentrates it, a high protein intake raises it, and bleeding into the gut raises it sharply. Because it responds to so much, it is more useful read next to creatinine than by itself.
In practice, UK reports print urea in mmol/L. American reports often print blood urea nitrogen (BUN) instead, which measures the same thing on a different scale, so the two numbers are not interchangeable.