An assay to assess thrombotic risk in which oxidized lipids comprising phospholipids are utilized as a membrane source in a clotting assay and the results compared to an assay in which unoxidized phospholipid is used as a membrane source in the presence and absence of activated protein C ("APC"). The assay can monitor for the presence of antibodies in the patient which interfere specifically with the anticoagulant function of APC in an oxidation dependent or independent manner. This can indicate the propensity of the patient to experience episodes of vein thrombosis or arterial thrombosis.