Bridging the Gap Between Chemiluminescence Immunoassay and Liquid Chromatography-Tandem Mass Spectrometry for Infliximab Monitoring: Method Validation and Clinical Comparability Study.

Infliximab (IFX) is a cornerstone biologic for pediatric inflammatory bowel disease, where therapeutic drug monitoring (TDM) is essential but complicated by inter-platform variability. Systematic comparison of automated immunoassays with reference methods is therefore warranted. This study aimed to validate an in-house chemiluminescence immunoassay (CLIA) for IFX quantification and evaluate its agreement with liquid chromatography-tandem mass spectrometry (LC-MS/MS). A CLIA kit was validated by assessing the limit of blank, specificity, linearity, accuracy, precision, matrix equivalence, and stability. IFX concentrations in 52 pediatric plasma samples were measured by CLIA and a reference LC-MS/MS method. Comparability was evaluated using linear regression, Passing-Bablok regression, and Bland-Altman analysis. CLIA showed excellent performance (lower limit of quantification: 0.640 µg/mL; specificity <10.0% interference; reportable range: 0.640-462 µg/mL). LC-MS/MS yielded higher concentrations than the CLIA (median: 6.25 vs. 2.87 µg/mL). Strong correlation was observed (r = 0.9101) with no significant deviation from linearity (Passing-Bablok, p > 0.05). However, Bland-Altman revealed substantial mean relative bias (67.9%) and poor categorical agreement (52% concordance) within the therapeutic range (3-7 µg/mL), indicating non-interchangeability. We validated a rapid, automated CLIA suitable for high-throughput TDM. Despite strong correlation, significant quantitative differences preclude direct result substitution, underscoring the need for method-specific therapeutic thresholds.