BACKGROUND Antiphospholipid syndrome (APS) is an autoimmune disease characterized by antibodies directed against phospholipids on plasma membranes. Through unclear mechanisms, APS confers hypercoagulability. APS may cause recurrent thromboses in the arterial and venous vasculature. We report a case of primary APS resulting in cerebral venous thrombosis and ST-elevation myocardial infarction (STEMI) for which only antiphosphatidylserine (aPS) IgM antibody was positive after extensive investigation. CASE REPORT A 48-year-old male was admitted after a witnessed generalized seizure with subsequent confusion. Imaging demonstrated thrombosis of multiple central nervous system (CNS) sinuses, including the superior sagittal sinus and bilateral transverse sinuses. The patient was heparinized with aggressive hydration, which proved inadequate, prompting endovascular thrombectomy. Three months later, despite anticoagulation therapy, the patient developed a STEMI when International Normalized Ratio (INR) was 1.8. Echocardiogram (ECHO) and PAN CT scan were normal. Initial coagulation studies demonstrated normal anticardiolipin antibody, prothrombin time, partial thromboplastin time, and platelet count. Outpatient coagulation studies revealed normal antithrombin III, protein C/S, hemoglobin electrophoresis, homocysteine, anti-β2 glycoprotein 1 antibodies, and D-Dimer. Factor V Leiden, JAK 2 mutation, prothrombin gene mutation, and tests for paroxysmal nocturnal hemoglobinuria (PNH) were negative. A positive phosphatidylserine IgM was detected. The patient was continued on warfarin (10 mg daily) with a target INR of 3.0-3.5 and clopidogrel (75 mg daily). CONCLUSIONS Despite extensive investigation, this patient only showed evidence of elevated aPS IgM antibodies, likely contributing to his CNS venous sinus thromboses and STEMI. It is important to screen for antiphosphatidylserine antibodies in cases of unprovoked thrombosis when standard thrombophilia analysis is unrevealing. This will assist in identifying pathogenicity and help prevent recurrence of subsequent thromboses.