PB1111 - Lupus Anticoagulant diagnosis: real life rate of interferences due to anticoagulant treatment in a Laboratory of a high complexity hospital.

Background: The presence of a Lupus Anticoagulant (LA) is one of the antiphospholipid syndrome laboratory criteria and a thrombosis risk factor. Studying patients during anticoagulant treatment is a challenge that makes the diagnosis very difficult.

Aims: To evaluate the rate of samples studied for LA at laboratory of Hospital Italiano de Buenos Aires in which anticoagulant treatment interferences made the results non interpretable (NIR).

Methods: Retrospective descriptive cross-sectional study. Period 2019-2021.
Follow-up was carried out until May 2022 for repetitions in patients with
NIR due to the presence of anticoagulants
LA testing: diluted Russell Viper Venom Time (HemosIL DRVVT screen/confirm), Silica Clotting time (HemosIL SCT screen /confirm), PT, APTT and Thrombin Time in ACL TOP coagulometer (Werfen). Diagnostic performed according to ISTH 2020 guidelines with local cut off points.

Results: Flow chart in figure 1, 87/2977 samples of different patients (3%) presented NIR due to anticoagulants treatment: 25 vitamin K antagonists (VKA), 20 low molecular weight heparin (LMWH), 20 direct oral anticoagulants (DOAC), and 22 from other laboratories without clinical data with elevated INR or antiXa activity in plasma were excluded. Probable false positive results in 26/87 (30%). Only 23/65 institutional patients underwent study repetition (35.4%). After anticoagulant discontinuation: 4 (17.4%) confirmed LA positive results, one with high titre of solid phase aPL antibodies and 3 negatives, 11 (47.8%) LA became negative. In 8/23 presented NIR because they were under anticoagulation with the same or other drug.

Conclusion(s): This real-life study demonstrated that in a centre where a very intensive teaching activity on this topic was done at least 3% of samples presented NIR due to anticoagulant treatment. Given the probability of false positive LA results, it is strongly recommended to carry out it after adequately anticoagulants suspension, since the repetition under the same or another anticoagulant will throw again NIR.