Investigation of cholesterol bias due to a matrix effect of external quality assurance samples: how true is your cholesterol method?

Background: Comparability of cholesterol measurement is clinically required and external quality assurance (EQA) programmes are important to verify the trueness of routine methods. Methods: We developed a gas chromatography-isotope dilution mass spectrometry (GC-IDMS) total cholesterol assay to investigate the cause of a suspected matrix-related negative bias with the Beckman Coulter enzymatic method discovered in an EQA programme. The GC-IDMS method was calibrated with certified reference material and verified against a secondary reference method. Bias between the GC-IDMS and Beckman Coulter methods was estimated according to Clinical and Laboratory Standards Institute (CLSI) protocol EP9-A2 with 40 clinical samples. Results: At clinically important decision levels, no significant bias was demonstrated on patients' samples (all results within a +/- 3% limit). A matrix effect confined to the EQA material that affected the Beckman Coulter total cholesterol method was confirmed. Conclusions: The GC-IDMS method is suitable as a higher order total cholesterol method in a routine clinical laboratory. Matrix effects defeat the objectives of EQA schemes by preventing the verification of trueness. Given the importance of obtaining a true cholesterol result without systematic error, we recommend that EQA material without matrix effects should be used. Ann Clin Biochem 2012; 49: 538-541. DOI: 10.1258/acb.2012.012009