What is cystatin C?
Cystatin C is a 13 kD, non-glycosylated, basic protein that is produced by all nucleated cells. It is freely filtered by the glomerulus and then reabsorbed and catabolized by the tubular epithelial cells, with only small amounts excreted in the urine. Its urinary clearance cannot be measured, which makes it difficult to study factors affecting its clearance and generation. The generation of cystatin C appears to be less variable and less affected by age and sex than serum creatinine; however, some studies have reported increased cystatin C levels associated with higher levels of C-reactive protein or BMI, hyperthyroidism and steroid use. In addition, other studies suggest extra-renal elimination at high levels of cystatin C and higher intra-individual variation compared to serum creatinine, particularly among transplant patients.
