ObjectivesAlthough a significant proportion of patients with cyanotic congenital heart disease are thrombocytopaenic, its prevalence and clinical significance in adults with Eisenmenger syndrome (ES) is not well studied. Accordingly, we examined the relationship of thrombocytopaenia and mean platelet volume (MPV) to bleeding or thrombotic complications and survival in a contemporary cohort of patients with ES, including patients with Down syndrome.MethodsDemographics, laboratory and clinical data were analysed from 226 patients with ES under active follow-up over 11 years.ResultsAge at baseline was 34.6±11.4 years and 34.1% were men. Mean platelet count and MPV were 152.6±73.3×10⁹/L and 9.6±1.2 fL, respectively. A strong inverse correlation was found between platelet count and haemoglobin concentration and MPV. During the study, there were 39 deaths, and 21 thrombotic and 43 bleeding events. On univariate Cox regression analysis, patients with a platelet count <100×10⁹/L had a twofold increased mortality (HR 2.10, 95% CI 1.10 to 4.01, p=0.024). Platelet count was not associated with an increased risk of thrombosis. However, there was a threefold increased thrombotic risk with MPV >9.5 fL (HR 3.50, 95% CI 1.28 to 9.54, p=0.015). Patients with either severe secondary erythrocytosis (>220g/L) or anaemia (<130g/L) were at higher risk of thrombotic events (HR 3.93, 95% CI 1.60 to 9.67, p=0.003; and HR 4.75, 95% CI 1.03 to 21.84, p=0.045, respectively).ConclusionsThrombocytopaenia significantly increased the risk of mortality in ES. Furthermore, raised MPV, severe secondary erythrocytosis and anaemia, but not platelet count, were associated with an increased risk of thrombotic events in our adult cohort.