Activated Factor X-Based versus Thrombin-Based Antithrombin Testing in Thrombophilia Workup in the DOAC Era

Area of Science:

• Clinical Chemistry
• Hematology
• Thrombosis Research

Background:

• Antithrombin (AT) activity testing is essential for diagnosing hereditary AT deficiency, a key genetic factor in thrombophilia.
• Two primary methods exist: those inhibiting thrombin (FIIa) and those inhibiting activated factor X (FXa).
• FXa-based assays are often favored for potentially higher sensitivity to specific AT deficiency mutations.

Purpose of the Study:

• To compare the real-world performance of FIIa-based and FXa-based antithrombin activity assays.
• To evaluate the impact of direct-acting oral anticoagulants (DOACs) and heparins on assay agreement.
• To determine the optimal assay choice for AT deficiency testing in various clinical contexts.

Main Methods:

• Analysis of 745 consecutively collected patient samples referred for thrombophilia testing.
• Comparison of AT activity results from FIIa-based and FXa-based assays.
• Stratification of samples based on anticoagulant use: no anticoagulants, low-molecular-weight heparin (LMWH), rivaroxaban, or apixaban.

Main Results:

• Good agreement between FIIa and FXa assays in patients without anticoagulants (r=0.874) and those on LMWH (r=0.891).
• Lower agreement observed in patients taking rivaroxaban (r=0.570) and apixaban (r=0.735).
• Direct FXa inhibitors, but not LMWH, dose-dependently affected FXa-based assay results, while FIIa-based tests remained unaffected.
• Both assay types successfully identified 90% of patients with hereditary AT deficiency mutations.

Conclusions:

• FXa-based AT testing is preferable to FIIa-based methods only when direct FXa inhibitors are absent.
• In patients receiving direct FXa inhibitors, FIIa-based assays are recommended for accurate AT activity testing.
• Both methods demonstrated comparable efficacy in detecting hereditary AT deficiency in the studied population.