Creatinine

Creatinine is a breakdown product of creatine phosphate following the creatine kinase reaction. Its significance in medicine is its use as a marker of kidney excretory function; more specifically, it's used to calculate the estimated glomerular filtration rate (eGFR), an estimate of the true glomerular filtration rate (GFR) when evaluating the kidney function. Creatinine and estimated glomarular filtration rate are extensively used in the evaluation of chronic kidney disease (CKD) and acute kidney injury (AKI).

The normal range of creatinine is approximately 60 - 100 µmol/L.

Physiology

Creatinine is eliminated by the kidneys by glomerular filtration (and to a smaller degree tubular secretion). It is normally not reabsorbed in the tubuli, except when there is oliguria. The rate at which blood plasma is cleared of creatine is called the creatinine clearance. Because creatinine is freely filtered in the glomeruli and is only secreted in the tubuli to a small degree, the creatinine clearance is approximately equal to the glomerular filtration rate.

1-2% of free creatine phosphate in skeletal muscle is broken down to creatinine daily. The rate of production depends on the muscle mass of the individual; the higher the mass, the higher the production. Serum creatinine levels are approx 30% higher in the evening than in the morning, likely due to muscle movement during the day.

Estimated glomerular filtration rate

See main article: Kidney function tests