An Overview of Heparin Monitoring with the Anti-Xa Assay

Heparin remains a critical therapy in hospitalized patients requiring anticoagulation. Unfractionated heparin (UFH) mediates its therapeutic effect by binding to antithrombin (AT) and inhibiting thrombin and FXa, as well as other serine proteases. Because of its complex pharmacokinetics, monitoring UFH therapy is required, which is usually achieved with either the activated partial thromboplastin time (APTT) or the anti-factor Xa (anti-Xa) assay. Low molecular weight heparin (LMWH) is fast replacing UFH, as it has a more predictable response, negating the need for routine monitoring in most cases. When required, the anti-Xa assay is used for monitoring of LMWH. The APTT has many notable limitations when used for heparin therapeutic monitoring, including biologic, preanalytical, and analytical issues. With its increasing availability, the anti-Xa assay is appealing as it is less affected by patient factors (e.g., acute-phase reactants, lupus anticoagulants, consumptive coagulopathies), known to interfere with the APTT. The anti-Xa assay has shown additional benefits, such as faster time to achieve therapeutic levels, more consistent therapeutic levels, less dose adjustments, and, overall, less tests performed during therapy. However, poor interlaboratory agreement has been observed among anti-Xa reagents, highlighting that further work needs to be done to standardize this assay for use in patient heparin monitoring.