Objectives Loss of positivity of antiphospholipid antibodies has been observed in clinical practice post thrombosis with secondary APS. Our study aimed to define the frequency of this loss.

Methods In this prospective study, the inclusion criteria comprised of SLE patients having at least two positive aPL markers in 3 years prior to the thrombosis. Patients with at least two post-thrombosis visits were included. Positive antiphospholipid markers comprised of RVVT>45, aCL IgG ≥20, aCL IgM ≥20 and aCL IgA ≥20. Loss of aPL was defined as being negative for these markers for all visits after the thrombosis, excluding the first visit post thrombosis. Percentages of loss of antiphospholipid markers after thrombosis was calculated. Further analyses were done for different types of thromboses (arterial vs venous). There were 17 patients with arterial and 16 patients with venous thromboses.

Results The analysis included the numbers and percentages of patients with loss of aPL after thrombosis, as shown in the table below.

Conclusions In secondary APS due to SLE, loss of aPL positivity post thrombotic event was up to 43% for anticardiolipin IgG, IgM and IgA, but only 13% for lupus anticoagulant (measured by RVVT). Loss of lupus anticoagulant positivity appears rare. Our analyses, however, are limited due to the small numbers of prospectively followed events.