It is uncertain whether reactive
thrombocytosis is associated with an increased risk of
thrombosis. This prospective case-control study assessed the in vitro thrombotic tendency of patients with reactive
thrombocytosis. Forty-eight patients with reactive
thrombocytosis, defined by platelet count >500x10(9)/l and 55 similar, randomly selected
critically ill patients who did not have reactive
thrombocytosis were considered. In vitro thrombotic tendency in both groups of patients was assessed using maximal amplitude (normal range 54 to 72 mm) and alpha angle (normal range 47 to 74°) on the thromboelastograph. The associations between reactive
thrombocytosis and
C-reactive protein, the coagulation profile and Sequential Organ Failure Assessment score were also evaluated. Patients with reactive
thrombocytosis had an associated increased in vitro thrombotic tendency (maximal amplitude 77 vs 69 mm, mean difference 8 mm, 95% confidence interval 4.9 to 10.9, P=0.001), a higher
fibrinogen concentration (7.2 vs 5.8 g/l, P=0.003), and a higher incidence of
infection requiring
antibiotics (50 vs 27%, P=0.025) compared to patients without
thrombocytosis. Platelet count had a relatively linear relationship with the maximal amplitude and the alpha angle of the thromboelastograph tracing (Pearson correlation coefficient: 0.53, P=0.001). In the multivariate analysis, only reactive
thrombocytosis (odds ratio 5.9, 95% confidence interval 1.3-27.8, P=0.025) and activated partial thromboplastin time (odds ratio 0.93 per second increment, 95% confidence interval 0.87 to 0.99, P=0.016) were significantly associated with a strong in vitro thrombotic tendency. In summary, reactive
thrombocytosis was associated with
infection requiring
antibiotics and evidence of increased in vitro thrombotic tendency in
critically ill patients.