Calibration of the Siemens Cystatin C Immunoassay Has Changed over Time

Abstract

To the Editor: The Siemens cystatin C immunoassay has been widely used in clinical research, particularly in the US. In recent years, however, the results obtained with the method appear to have changed. The glomerular filtration rate (GFR) is generally accepted as the best overall indicator of kidney function and is an important measure for assessing kidney disease. Several studies have shown cystatin C to be superior to creatinine for estimation of the GFR (1), which is usually expressed as the relative GFR [in units of mL · min−1 · (1.73 m2)−1]. This practice has led to the development of formulas to convert cystatin C measurements in milligrams per liter to a calculated GFR in these units, without the need for demographic coefficients (2, 3). The formulas were developed from studies that compared cystatin C concentrations with measured GFRs by using such exogenous markers as iohexol, diethylenetriamine pentaacetic acid or 51Cr-EDTA clearance (4). We were concerned that the calibration of the Siemens cystatin C method had changed during the last 5 years, because we noted that the cystatin C concentrations of participants in a longitudinal cohort improved substantially …