Urea

The waste product your liver makes from used-up protein, and your kidneys clear.

It moves with kidney function like creatinine does, but it is also sensitive to things the kidneys have nothing to do with, which is what makes the pair informative.

It rises with dehydration, a high-protein diet and bleeding into the gut, so a raised urea with a normal creatinine usually points at one of those rather than at the kidneys. Many labs print blood urea nitrogen instead, which is the same measurement on a different scale.

Worth discussing a rise that holds alongside creatinine.