Cystatin C: a promising marker and predictor of impaired renal function.

Cystatin C is a relatively stable protein in serum and heparinized plasma that shows promise as a convenient measure of glomerular filtration rate (GFR). However, it is becoming clear that the relationship between cystatin C and GFR can depend on the clinical presentation. Factors influencing cystatin C levels are those that affect the rate of synthesis of the protein, such as thyroid status and the use of steroids. As with all laboratory tests, results should be interpreted in the light of the method's known limitations and in conjunction with other clinical and laboratory information. Nevertheless, accumulating evidence suggests that cystatin C is a useful biomarker for renal function, and may even be the method of choice in a range of clinical situations, from GFR surveillance in diabetics to the assessment of acute kidney injury in critically ill patients.