Creatinine clearance

Accurate measurement of glomerular filtration rate by a clearance test requires determination of the concentrations in plasma and urine, of a substance which is freely filtered at the glomerulus, and is neither reabsorbed nor secreted by the renal tubules. Its concentration in plasma should remain constant throughout the period of urine collection.

Creatinine is often used to assess GFR although it does not fulfill all the above requirements. It is freely filtered, but a small amount is reabsorbed, and a larger quantity, up to 10% of urinary creatinine, results from tubular secretion. As GFR falls, creatine clearance deviates further from the true GFR.

A normal creatinine clearance is 100-150 ml/min. It may reach 200 ml/min in pregnancy. A clearance of less than 80 ml/min is significant except in people over 80 years.

Notes:

• there is no need to collect 24 h urine samples to measure creatinine clearance in primary care (1). Kidney function in patients with chronic kidney disease should be assessed by formula-based estimation of GFR (1)

Reference:

1. The Renal Association (May 2006).UK CKD Guidelines.