Clinical and Interpretive

Urinary creatinine, in conjunction with serum creatinine, is used to calculate the creatinine clearance, a measure of renal function.

Creatinine is formed from the metabolism of creatine and phosphocreatine, both of which are principally found in muscle. Thus the amount of creatinine produced is in large part dependent upon the individual’s muscle mass and tends not to fluctuate much from day-to-day.  Creatinine is not protein-bound and is freely filtered by glomeruli. All of the filtered creatinine is excreted in the urine. Renal tubular secretion of creatinine also contributes to a small proportion of excreted creatinine. Although most excreted creatinine is derived from an individual’s muscle, dietary protein intake, particularly of cooked meat, can contribute to urinary creatinine levels.The renal clearance of creatinine provides an estimate of glomerular filtration rate.

24-Hour urinary creatinine determinations are principally used for the calculation of creatinine clearance.

Decreased creatinine clearance indicates decreased glomerular filtration rate. This can be due to conditions such as progressive renal disease, or result from adverse effect on renal hemodynamics that are often reversible including certain drugs or from decreases in effective renal perfusion (eg, volume depletion or heart failure).

Increased creatinine clearance is often referred to as “hyperfiltration” and is most commonly seen during pregnancy or in patients with diabetes mellitus, before diabetic nephropathy has occurred. It also may occur with large dietary protein intake.

The reliability of 24-hour urinary creatinine determinations are, as for all timed urine collections, very dependent on accurately collected 24-hour specimens.  Intraindividual variability in creatinine excretion may be due to differences in muscle mass or amount of ingested meat.

Acute changes in GFR, before a steady state has developed, will alter the amount of urinary creatinine excreted.