Absence of Circadian Variations in Urine Cystatin C Allows Its Use on Urinary Samples

Cystatin C (CST3), which belongs to the type II cystatin gene family, is a nonglycosylated 13-kDa protein constitutively secreted shortly after its synthesis (1). Because its low molecular mass and its positive charge at physiologic pH allow it to be freely filtered by kidney glomeruli and because CST3 normally is reabsorbed and then almost completely catabolized by proximal tubular cells (2), CST3 that was not metabolized is eliminated in urine and may represent a useful marker of tubular injury (3) or renal tubular dysfunction (4). The quantitative CST3 assay we developed is highly reliable and may be used as a part of the standard screening panel for renal failure. However, the use of 24-h urine samples is time-consuming and fails to offer clinically relevant data regarding tubular status in emergency cases. To evaluate whether a full 24-h collection is necessary, we analyzed, over a 24-h period, CST3 urinary release to see whether circadian variations were evident. We recruited 11 healthy individuals and collected urine samples every 2 h during a 24-h …