The study focused on monitoring unfractionated heparin treatment using two tests: APTT and chromogenic anti-factor Xa. The researchers developed and validated according to CLSI guidelines a proprietary chromogenic anti-Factor Xa assay that showed very good agreement with the commercial platform (Pearson r = 0.986, R² = 0.972). In 110 patients (190 samples), they established a laboratory-specific therapeutic APTT range of 79–127 seconds at a target anti-Xa range of 0.3–0.7 IU/ml. There was only a moderate correlation between APTT and anti-factor Xa (Spearman ρ = 0.678, p < 0.001). Up to 58.2% of samples had a therapeutic level classification discrepancy between APTT and anti-Xa. Using a random forest model (AUC = 0.79) and logistic regression, they identified five factors associated with this discrepancy: fibrinogen, triglycerides, estimated glomerular filtration rate, C-reactive protein, and antithrombin III. The study showed that, despite a validated anti-Xa assay, there is significant discordance between heparin monitoring by APTT and anti-factor Xa.