Background: The long-term outcome of individuals with mild degrees of thrombocytopenia is unknown.
Methods and findings: In a prospective study conducted between August 1992 and December 2002, 260 apparently healthy individuals with incidentally discovered platelet counts between 100 x 10(9)/l and 150 x 10(9)/l were monitored for 6 mo to determine whether their condition persisted. The monitoring period was completed in 217 cases, of whom 191 (88%) maintained stable platelet counts. These 191 individuals were included in a long-term follow-up study to gain knowledge of their natural history. With a median time of observation of 64 mo, the thrombocytopenia resolved spontaneously or persisted with no other disorders becoming apparent in 64% of cases. The most frequent event during the study period was the subsequent development of an autoimmune disease. The 10-y probability of developing idiopathic thrombocytopenic purpura (ITP), as defined by platelet counts persistently below 100 x 10(9)/l, was 6.9% (95% confidence interval [CI]: 4.0%-12.0%). The 10-y probability of developing autoimmune disorders other than ITP was 12.0% (95% CI: 6.9%-20.8%). Most of the cases (85%) of autoimmune disease occurred in women.
Conclusions: Healthy individuals with a sustained platelet count between 100 x 10(9)/l and 150 x 10(9)/l have a 10-y probability of developing autoimmune disorders of 12%. Further investigation is required to establish whether this risk is higher than in the general population and whether an intensive follow-up results in an improvement of prognosis.