Objective: The objective of this study was to identify risk factors for developing chronic idiopathic thrombocytopenic purpura in a cohort of pediatric patients with idiopathic thrombocytopenic purpura.

Methods: We conducted a retrospective cohort analysis of 259 children who were diagnosed with idiopathic thrombocytopenic purpura between 1991 and 2000 at 1 of 8 managed care organizations that comprise the Vaccine Safety Datalink. We reviewed the charts of 595 potential patients with idiopathic thrombocytopenic purpura from the 8 Vaccine Safety Datalink sites and excluded patients with known causes of thrombocytopenia. Chronic idiopathic thrombocytopenic purpura was defined as having thrombocytopenia for 6 months beyond the initial diagnosis. The risk for developing chronic idiopathic thrombocytopenic purpura was assessed using simple and multivariable analyses.

Results: Of the 259 cases of idiopathic thrombocytopenic purpura, 197 (76%) were acute, 60 (23%) were chronic, and 2 (1%) could not be determined. Among the acute cases, the mean duration of illness was 22 days. There was 1 serious bleeding outcome in the cohort. In multivariable regression analysis, the patients with chronic illness were older, less likely to present with mucosal bleeding, less likely to have had an acute illness before diagnosis, and more likely to present with a platelet count > 20,000/microL than children with acute idiopathic thrombocytopenic purpura. In particular, children whose illness was diagnosed at > or = 10 years of age and who had platelet counts > or = 20,000/microL had an approximate fivefold risk for progressing to chronic disease when compared with children who presented at < or = 2 years of age with platelet counts < 20,000/microL.

Conclusions: Although idiopathic thrombocytopenic purpura tends to be a benign and self-limited condition, acute and chronic idiopathic thrombocytopenic purpura seem to be distinct illnesses defined by age, platelet count, bleeding symptoms, and the presence of acute illness before diagnosis. Physicians should be aware of these differences when advising their patients and families.