Long-Term Performance of Laboratory-Developed Liquid Chromatography–Tandem Mass Spectrometry Tests and a Food and Drug Administration–Approved Immunoassay for the Therapeutic Drug Monitoring of Everolimus

Laboratory-developed tests (LDTs) are analytical tests developed and validated "in-house" for clinical diagnosis. Regulatory agencies, such as the United States Food and Drug Administration (FDA), encourage using regulatory-approved assays rather than LDTs. In the ongoing Zortracker everolimus study, samples were provided monthly to participating clinical laboratories that conduct therapeutic drug monitoring of everolimus. This allowed for the long-term (6-year) comparison of the performance of liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays, which are LDTs, to the FDA-approved everolimus Quantitative Microsphere System (QMS).Each laboratory received the same 3 blinded samples. LC-MS/MS and QMS assays were compared using Passing Bablok regression analysis. Data were analyzed in 12-month periods to detect trends over time.The slopes of the Passing Bablok regression curves remained unchanged in 2013 and 2014 (reference LC-MS/MS; test QMS: slope = 0.934 and 1.008). However, by 2016, the slope increased significantly to between 1.111 and 1.320, then dropped to 0.980 in 2017 and 0.912 in 2018, suggestive of changes in QMS bias compared with LC-MS/MS over longer periods. Outliers did not affect these results. The interlaboratory variability of LC-MS/MS and QMS remained unchanged from 2013 to 2015, with coefficients of variation of 15.3%, 18.4%, and 17.2% for LC-MS/MS, and 13.0%, 13.1%, and 15.3% for QMS, respectively, per year. At the end of the observation period, the average coefficients of variation in LC-MS/MS laboratories dropped to 14.3%, 12.6%, and 14.2%, whereas the variability in QMS laboratories was 19.5%, 13.3%, and 19.6% in 2016, 2017, and 2018, respectively.Initially, QMS everolimus concentrations in patient samples were comparable with those detected in LC-MS/MS laboratories. However, concentration bias of the QMS assay significantly changed within 6 years, emphasizing the need for long-term, independent performance tracking of therapeutic drug monitoring assays, including FDA-approved assays.