Improved Performance in Measurement of Serum Cystatin C by Laboratories Participating in the College of American Pathologists 2019 CYS Survey

• Context.-Use of cystatin C for glomerular filtration rate estimation (eGFR) has garnered heightened interest as a means to avoid race-based medicine, since eGFR_cys equations do not require specification of race. Before considering more widespread use of cystatin C, it is important to confirm that assays provide accurate measurements of cystatin C concentration, to ensure accurate GFR estimates. Objective.-To determine if the accuracy of cystatin C measurements in laboratories participating in the College of American Pathologists (CAP) Cystatin C (CYS) survey has improved since 2014. Design.-Two fresh frozen serum pools, the first from healthy donors without chronic kidney disease (CKD), and the second from patients with CKD, along with a synthetically prepared elevated cystatin C pool, were sent to laboratories participating in the 2019 CYS-A survey. Target values were established by using 2 immunoassays and a bracketed 2-point calibration with diluted ERMDA471/IFCC reference material. Results.-For the healthy donor fresh frozen pool (ERMDA471/IFCC-traceable target of 0.725 mg/L), the all-method mean (standard deviation, coefficient of variation) was 0.731 mg/L (0.071, 9.7%). For the CKD pool (ERMDA471/IFCC-traceable target of 2.136 mg/L), the all-method mean was 2.155 mg/L (0.182, 8.4%). For the synthetically spiked pool (ERM-DA471/IFCC-traceable target of 1.843 mg/L), the all-method mean was 1.886 mg/L (0.152, 8.1%). This represents marked improvement in accuracy and between-method agreement compared to the 2014 CAP survey. Conclusions.-Manufacturers have markedly improved accuracy and between-method agreement of cystatin C measurement procedures since 2014, which allows for greater confidence in estimated GFR relying on cystatin C.