A Targeted LC-MS/MS-Based Quantitative Assay for Detecting Plasma Factor XIII A/B Subunit Deficiency

Factor XIII (FXIII) is a heterotetramer that plays a crucial role in the coagulation cascade, facilitating fibrin crosslinking to stabilize clots. Congenital or acquired deficiency of the FXIII A or B subunits can lead to prolonged bleeding. Existing enzyme-linked immunosorbent assay-based and latex agglutination assays for measuring FXIII are limited to detecting only the FXIIIA subunit of the complex. In this study, a targeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay was developed and validated to accurately quantify both FXIIIA and FXIIIB subunits in plasma. Peptides measured by this targeted assay were rigorously selected based on analytical robustness, with the method demonstrating excellent sensitivity (limit of detection, 1.56 to 3.67 mU/mL), linearity (r² > 0.998), and precision (CV, <10%). Comparison with the conventional assay based on ammonia release demonstrated a strong correlation (r = 0.983) and agreement (Cohen κ = 0.846) for FXIIIA. On analysis of 98 clinical plasma samples, the assay accurately identified cases with FXIII deficiency based on significantly reduced FXIIIA with varying alterations in FXIIIB levels. These results establish the clinical utility of the LC-MS/MS assay, highlight its potential for improving diagnostic accuracy for FXIII deficiency, and lay the foundation for future research on FXIII dynamics in pathologic states.