Creatinine, 24-Hour Urine

The Creatinine, 24-Hour Urine test is used to assess renal excretory function and to monitor patients with established or suspected kidney disease. As a companion measure to other timed urine studies, 24-hour urine creatinine helps verify whether a collection is complete and suitable for analysis. When interpreted with a concurrent serum creatinine, urine creatinine from a timed collection can be used to estimate creatinine clearance.

Creatinine is produced from creatine and phosphocreatine, mainly in skeletal muscle, during energy-generating processes that support contraction. The kidneys eliminate creatinine with minimal tubular reabsorption, yielding urinary concentrations that are tens to hundreds of times higher than those in blood. Within an individual, day-to-day creatinine excretion is relatively stable and is proportional to muscle mass, leading to differences by sex and age. Paired measurement of serum creatinine and 24-hour urine creatinine permits calculation of creatinine clearance as an estimate of glomerular filtration rate; reduced renal function lowers the amount of creatinine excreted in the timed specimen. Because timed urine creatinine output is comparatively steady, it is used to judge the adequacy of 24-hour urine collections and is frequently ordered with urine protein or albumin in the evaluation of kidney disease.