Creatinine

The test most frequently used to evaluate renal function is the creatinine assay in serum or plasma. A byproduct of creatine phosphate breakdown in muscle, creatinine is typically produced by the body at a fairly steady rate (dependent on muscle mass). Under normal circumstances, it is not significantly reabsorbed by the tubules after being freely filtered by the glomeruli. Additionally, a tiny but substantial amount is actively secreted.