Renal Function Tests and Creatinine Clearance

Key points

• • The kidneys clear nitrogenous waste, so a rising blood urea and creatinine are the simplest markers of falling kidney function.
• • Clearance is the volume of plasma completely cleared of a substance in a unit of time, and it estimates the glomerular filtration rate.
• • Creatinine clearance is widely used because creatinine is produced steadily from muscle and is filtered but only slightly secreted by the tubules.
• • An ideal filtration marker is freely filtered and neither reabsorbed nor secreted; inulin fits this best, while creatinine slightly overestimates the rate.
• • Blood urea rises not only in kidney disease but also with dehydration, a high-protein diet and gastrointestinal bleeding, so it is less specific than creatinine.