Antiphospholipid syndrome without antiphospholipid antibodies at the time of the thrombotic event: transient 'seronegative' antiphospholipid syndrome?

The antiphospholipid syndrome (APS) is characterized by the presence of venous and arterial thrombosis, recurrent fetal losses and thrombocytopenia, associated with the presence of antiphospholipid antibodies (aPL). This syndrome may be "primary" or may be associated with other diseases, mainly systemic lupus erythematosus (SLE). However, some patients present the clinical picture of this syndrome but without evidence of aPL in their serum. The term "seronegative" APS has been proposed to categorize these patients. Here with we present two patients with seronegativity for aPL at the time of a thrombotic event, but in whom these antibodies were detected 2 and 7 months later.