Differential Discussions Guest Series: The Use of Chromogenic Factor X Assays for Monitoring Patients on Warfarin Therapy 

Posted on: April 11, 2025

David L. McGlasson, MS, MLS(ASCP) 

A recent episode of “Differential Discussions” digs into the utility of the chromogenic factor X (CFX) assay for monitoring patients on oral anticoagulant therapy (OAT). Data from previous studies and a case study were discussed.

A study was presented that compared International Normalized Ratio (INR) values with CFX results, to determine normal, subtherapeutic, therapeutic, and supratherapeutic ranges for patients in an anticoagulation clinic. Specimens were categorized as normal, OAT patients with INR <2.0, INR 2.0-3.0, and INR >3.0. The statistics from the study were discussed, suggesting that the CFX assay can be a valuable tool for monitoring OAT in patient populations where INR values are unreliable.

A case study was also discussed, which involved a patient with congenital heart disease who had a mechanical atrioventricular valve replacement. Despite receiving warfarin with INR at seemingly target ranges, the patient experienced acute valvular thrombosis. When the CFX was measured, the CFX levels did not align with the INR results, indicating a subtherapeutic level of anticoagulation despite being within the therapeutic INR range. The anticoagulation team decided to use CFX levels to interpret INR results and guide an individualized anticoagulation plan for this patient. The data suggests that for some patients, the anticoagulation team should consider using the CFX assay alongside INR to establish corrected ranges in the treatment plan.

View the full episode below or on YouTube.

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