ACTIVATED PROTEIN-C RESISTANCE PHENOTYPE IN PATIENTS WITH ANTIPHOSPHOLIPID ANTIBODIES

The effect of antiphospholipid antibodies (aPL) on the action of activ ated protein C (APC) was examined in 32 patients: 19 with lupus antico agulant (LA), 6 with anticardiolipin antibodies (aCL), and 7 with LA a nd aCL Eighteen patients had a ratio of activated partial thromboplast in time (APTT) with APC to APTT without APC (APTT) ratio) < 2.06 (cut- off level) and no factor V Leiden Mutation; these patients showed APC- resistance (APC-R) phenotype. the mean prolongation of APTT after addi tion of APC in a control group was 45.3 seconds, with a lower limit of 31.4 seconds. Only 3 of the 18 patients with low APTT ratio had a pro longation of < 31.4 seconds; they were classified as true APC-R phenot ype, whereas the other 15 patients were classified as spurious APC-R. Of the 3 patients with true APC-R, 2 had deep venous thrombosis, 1 wit h pulmonary embolism, and the third had recurrent abortion. Of the oth er 15 patients, 2 had had ischemic stroke, 1 had recurrent abortion, a nd 12 were asymptomatic. Circulating APC level was measured in 14 of t he 18 aPL patients with a low APTT ratio; it was lower than the normal lower limit in 4 patients and within the lower limit in 2. three of t he 4 patients with reduced APC levels had a history of thrombosis. We conclude that patients with aPL who show APC-R phenotype due to a low APTT ratio without the factor V Leiden mutation can be classified into two groups: true and spurious APC-R phenotype. Since those with true APC-R phenotype could have greater thrombotic risk, adequate classific ation of these patients is important. Moreover, aPL can sometimes inte rfere with the activation of protein C, thus reducing the circulating levels of APC, and this could constitute another thrombotic risk facto r.