Evaluation Of The Current Status Of Serum Cystatin C Measurement Standardization And Establishment And Evaluation Of Isotope Dilution Liquid Chromatography Tandem Mass Spectrometr

Objectives:Serum Cystatin C(CysC)is often used in clinical practice to estimate glomerular filtration rate(GFR)and to assess renal function.To clarify the status of CysC assay standardization and to promote assay standardization,the first section of this study assessed the quality status of immunoassays frequently used in clinical laboratories and the commutability of multiple processed materials;the second section established and evaluated an isotope dilution liquid chromatography tandem mass spectrometry(ID-LCMS/MS)method for serum CysC.Methods:In the first section of this study,the agreement of seven CysC immunoassays claiming traceability was assessed using Passing-Bablok(P-B)regression and relative deviation plots,based on 43 single-donor serum samples;the commutability of reference material ERM-DA471/IFCC,its dilutions and other processed materials were assessed using the IFCC approach.After demonstrating commutability,the reference material was then used to assess the trueness of routine methods.In the second section,the ID-LCMS/MS method used a specific peptide for CysC quantification and a SIL-T3 peptide for internal calibration;the sample preparation procedure was optimized,as well as the mass spectrometry and chromatographic conditions;the precision,trueness,matrix effects and limit of quantification were evaluated;the method was also compared with immunoassays.Results:In the first section,P-B regression showed good correlation but significant proportional or constant deviation between routine methods.Using the average testing results of seven methods as target,the relative deviations of 4/7 methods were not constant over the measurement range and showed linear or irregular trends,with 49%-100%of the testing results meeting the bias limit.Frozen and lyophilized human serum and DA4712.34 were shown to be commutable in 8/8 method pairs.Trueness evaluation by DA4712.34 showed that BSBE had an unacceptable positive bias,while Roche and Beckman had a negative bias.In the second section,the ID-LC-MS/MS method had an analysis time of 9 min,a measurement range of 0.31-10.00 mg/L,a total CV of 1.73%-4.05%and a limit of quantification of 0.15 mg/L.Assay comparison demonstrated that the method correlated well with routine immunoassays.Conclusions:Unacceptable bias was observed for some CysC immunoassays claiming traceability and the agreement between immunoassays remains to be improved.Therefore,manufacturers should improve the calibration and traceability procedures.Frozen/lyophilized serum pools were commutable across immunoassays and could be used as candidate reference materials.The ID-LC-MS/MS method showed good performance and should be improved to become a candidate reference method.