Review Article: Antiphospholipid Syndrome as a Complicating Factor for INR Testing: A Review of the Literature

Kassandra Erickson, Emily Van Klompenburg

Abstract

Antiphospholipid syndrome is an autoimmune disorder associated with recurrent arterial and venous thrombosis requiring long-term anticoagulation, most commonly with warfarin. Monitoring warfarin therapy relies on accurate INR measurement; however, antiphospholipid antibodies may interfere with phospholipid-dependent coagulation assays, resulting in inaccurate INR values. This literature review summarizes current evidence regarding the impact of APS on INR testing methodologies, with emphasis on discrepancies between POC INR testing and venipuncture-based laboratory methods. A non-systematic review of the literature was conducted using PubMed and Web of Science to identify studies evaluating INR measurement accuracy, methodology, and anticoagulation monitoring in APS populations. The reviewed literature demonstrates that APS is a significant source of variability in INR testing due to interactions between antiphospholipid antibodies and thromboplastin reagents. POC INR testing appears most susceptible to interference, frequently producing falsely elevated INR results compared with laboratory-based methods. Quick’s method also demonstrates increased sensitivity to lupus anticoagulant interference, whereas Owren’s method appears less affected due to differences in thromboplastin composition and assay design. Alternative approaches, including chromogenic factor X assays and individualized correction strategies, may improve anticoagulation monitoring accuracy in APS patients. Overall, the findings highlight important limitations of phospholipid-dependent coagulation assays in APS and underscore the need for increased awareness among laboratory professionals and clinicians regarding assay-specific interference. Recognition of these limitations is essential to support accurate anticoagulation management, appropriate confirmatory testing practices, and improved patient safety.

Abbreviations: Antiphospholipid syndrome (APS); international normalized ratio (INR), point-of-care (POC)

Keywords: Antiphospholipid syndrome; international normalized ratio; point-of-care testing; warfarin monitoring; coagulation assays.

Int. J. Bio. Lab. Sci 2026(15)2:109-116 【PDF】