Announcement
Thank you for attending THSNA 2026. The virtual meeting is now closed.
Thank you for attending THSNA 2026. The virtual meeting is now closed.
Presentation Details
| Factor VIII Inhibitor Kinetics and Reagent Composition Underlie False-Positive Lupus Anticoagulant Patterns in Acquired Hemophilia A Hamish Nicolson1, 2, Steven Wong1, 2. 1Providence Health Care, St.Paul’s Hospital, Vancouver, BC, Canada.2Department of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, BC, Canada |
Abstract
Background Both lupus anticoagulant (LA) and acquired hemophilia A (AHA) cause isolated prolongation of activated partial thromboplastin time (APTT), but by distinct mechanisms. LA produces phospholipid-dependent inhibition of coagulation, whereas AHA results from autoantibody-mediated inhibition of factor VIII (FVIII). LA detection uses paired low and high phospholipid reagents. Differences in phospholipid concentration and activator type influence FVIII sensitivity and, in AHA, may produce LA-like screen–confirm patterns. These findings demonstrate that reagent composition and FVIII inhibitor kinetics interact to generate false-positive LA patterns. Methods Two retrospective datasets were analyzed. Dataset 1 included twelve AHA patients (low FVIII activity by chromogenic assay and positive Bethesda inhibitor titers). APTT-SynthASil (SIL) and APTT-SynthAFax (FAX) were available for nine patients, Silica Clotting Time screen/confirm (SCTs/SCTc) with normalized ratio (NR) results for five, and dilute Russell viper venom time (dRVVT) NR for seven. Dataset 2 included eighteen patients with congenital FVIII deficiency without inhibitors or von Willebrand disease. An in-vitro FVIII dilution series was performed using SIL, FAX, SCTs, and SCTc to characterize reagent responses to decreasing FVIII activity. Results In AHA, FVIII was <5% with mean differences 25s for SIL–FAX (range 10–47s) and 34s for SCTs–SCTc (range 10–73s) (Figure. 1). Two of five AHA cases (71 and 325 BU) had false-positive SCT NRs (1.35 and 1.54; cutoff >1.29) with normal dRVVT results. Congenital FVIII deficiency cases had mean FVIII 25% (range 4–49%) with mean differences of 7s for SIL–FAX (range 0–13s) and 9s for SCTs–SCTc (range 4–41s) (Figure. 2), mean SCT NR 0.95 and no false positives or screen–confirm divergence, even among cases with FVIII ≤11%. In-vitro FVIII dilution (6–100%) showed distinct reagent responses as FVIII declined; ULN thresholds occurred at FAX ≈57%, SCTc ≈55%, SIL ≈34%, and SCTs ≈19%. Discussion These findings show that reagent-specific responses to FVIII reduction create the potential for discordant clotting results, but the exaggerated screen–confirm pattern occurs only in AHA. The FVIII dilution series confirms that reagents exhibit non-identical clotting-time responses as FVIII falls, crossing ULN at different levels. Congenital FVIII deficiency produced proportionate prolongation across reagents, even among cases with FVIII ≤11%, indicating that low FVIII alone does not generate the discordance seen in AHA. We propose that non-linear FVIII inhibitor kinetics in AHA amplify these reagent-dependent differences. When FVIII is inhibited, formation of the intrinsic tenase complex (FIXa–FVIIIa–phospholipid–Ca²⁺) can become rate-limiting for clot generation, increasing the influence of reagent composition on clotting time and producing an apparent screen–confirm discrepancy despite the absence of phospholipid-dependent inhibition. Conclusion The LA-mimicking pattern in AHA arises from interaction between FVIII inhibitor kinetics and reagent composition, not low FVIII alone. Autoantibody-mediated FVIII inhibition amplifies reagent-dependent differences in clotting response, generating a false screen/confirm discrepancy mimicking phospholipid-dependent inhibition. Recognizing this diagnostic pitfall is crucial: reflex FVIII testing when an isolated prolonged APTT is accompanied by a normal dRVVT can prevent misclassification of AHA as LA and ensure timely diagnosis and management of this potentially life-threatening bleeding disorder.
No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.
No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.