In-house chromogenic anti-factor Xa assay: development, validation, and identification of factors predicting APTT discordance

Background: Activated partial thromboplastin time (APTT) is the conventional test for monitoring unfractionated heparin (UFH) therapy, but discordance with anti-factor Xa results frequently occurs in clinical practice, potentially leading to suboptimal anticoagulation management. Objective: This study aimed to develop an in-house anti-factor Xa detection system, establish laboratory-specific APTT therapeutic ranges calibrated against anti-factor Xa levels, and identify predictors of discordance between these two monitoring methods. Methods: We developed and validated a chromogenic anti-factor Xa assay according to CLSI guidelines. From 110 patients receiving UFH, 190 samples were collected. Laboratory-specific APTT ranges were derived using linear regression and ROC analysis against anti-factor Xa target range (0.3-0.7 IU/ml). Random forest and logistic regression models identified predictors of discordance among clinical and laboratory variables. Results: < 0.001), classification concordance was poor, with 58.2% of specimens showing discordance. Random forest analysis (AUC = 0.79) and logistic regression identified five key predictors: fibrinogen, triglycerides, estimated glomerular filtration rate, C-reactive protein, and antithrombin III. Conclusions: This study developed a validated anti-factor Xa assay and identified substantial classification discordance between APTT and anti-factor Xa monitoring. Five clinical and laboratory factors were associated with monitoring discordance.