A Nordic Population-Based Cohort Study of Risk of Arterial Thrombotic and Venous Thromboembolic Events in Patients with Primary Chronic Immune Thrombocytopenia (cITP)

Background: ITP is a rare autoimmune disorder characterized by low platelet counts, resulting in an increased risk for bleeding. Paradoxically, patients with cITP may also have an increased incidence of thrombotic or thromboembolic events, but population-based data on this are limited. We estimated the incidence of these events in a prospective cohort study of incident cITP patients in three Nordic countries.

Methods: Based on National Health Registry Systems (NHRSs) and medical records in Denmark, Norway, and Sweden, the study cohort included all adults diagnosed with cITP from January 1, 2008 to December 31, 2012 (n=1,821). Arterial thrombotic events (myocardial infarction [MI] and stroke) and venous thromboembolic events (pulmonary embolism and deep vein thrombosis) were identified in the NHRSs. Patients were followed from the latest of cITP diagnosis or April 1, 2009, until the earliest date of the first occurrence of the event of interest, death, emigration, or December 31, 2012. Incidence rates (events per 1,000 person-weeks [PW]) were computed for the entire cohort and stratified by gender, age, splenectomy status, and comorbidity burden.

Results: Nearly 60% of the cITP cohort was >50 years in age, and 56% were female. Overall, the incidence of arterial thrombotic events was 0.31 per 1,000 PW (95% confidence interval [CI]: 0.25-0.39) and the incidence of venous thromboembolic events was 0.18 per 1,000 PW (95% CI: 0.13-0.24). For arterial thrombotic events, the risk was higher in males (compared with females), and the risk for both event types increased with increasing age and comorbid burden. Given the small number of patients who underwent splenectomy (n=101), it was difficult to detect any differences in the risk of thrombotic events by splenectomy status, but the incidence of venous thromboembolic events was higher in splenectomized versus non-splenectomized patients (0.27 per 1,000 PW [95% CI: 0.15-0.50] versus 0.16 [95% CI: 0.11-0.23]).

Conclusions: Among patients with cITP, the risk of arterial thrombotic events is higher in males than females and increases with increasing age and level of comorbid burden. The risk of venous thromboembolic events is heightened in cITP patients who have undergone splenectomy.