Thrombotic risk stratification by platelet count in patients with antiphospholipid antibodies: a longitudinal study

Background Thrombocytopenia is a non-criteria clinical manifestation of antiphospholipid syndrome. However, it remains to be elucidated whether thrombocytopenia increases thrombotic risk in antiphospholipid antibody (aPL) carriers. Objectives To investigate the impact of platelet count in terms of predicting thrombotic events in aPL carriers, and to stratify the thrombotic risk by combining platelet count and antiphospholipid score (aPL-S), which represents a quantification of aPL varieties and titers. Patients/methods A single-center, retrospective, longitudinal study comprising 953 consecutive patients who were suspected of having autoimmune disease between January 2002 and December 2006 was performed. Low platelet count was defined as a count of < 150 x 10(3) L-1 at the time of aPL testing. Results A negative correlation was observed between aPL-S and platelet count (r = - 0.2477). Among aPL-positive patients, those with a low platelet count developed thrombosis more frequently than those without (hazard ratio [HR] 2.95, 95% confidence interval [CI] 1.11-7.88). Among aPL-negative patients, no difference was found in the predictive value of thrombosis regardless of platelet count. Patients with aPLs were further divided into two subgroups according to aPL-S. Among low-aPL-S patients, those with low platelet counts developed thrombosis more frequently than those without (HR 3.44, 95% CI 1.05-11.2). In contrast, high-aPL-S patients developed thrombosis frequently regardless of platelet count. Conclusions aPL carriers with low platelet counts are at high risk of developing thrombosis. In particular, low-aPL-S carriers' may be stratified by platelet count in terms of predicting future thrombotic events.