Mbernardo just posted an article about ITP which included statistics. Here they are:
ITP is defined as a platelet count of less than 100 × 109/L (100,000/μL) with no evidence of leukopenia or anemia. This cutoff point is new: in the past, ITP was defined as a platelet count of less than 150 × 109/L, which is the threshold for a normal platelet count in most laboratories.
The platelet threshold of 100 × 109/L was based on a study by Stasi et al,2 who followed 217 otherwise healthy people who had an incidental finding of mild thrombocytopenia (platelet count 100–150 × 109/L). Within 6 months, the platelet count rose to more than 150 × 109/L in 23, while three had either worsening thrombocytopenia or were diagnosed with other conditions. During longterm follow-up (median 64 months), 109 of the remaining 191 individuals remained stable, 13 developed counts greater than 150 × 109/L, 12 developed ITP, 13 developed an autoimmune disorder, 18 developed other disorders, and 26 were lost to follow-up. The 10-year probability of developing ITP, defined as a platelet count persistently below 100 × 109/L, was only 6.9%, indicating that the chances are small that a person with an isolated finding of mild, stable thrombocytopenia will develop ITP.
www.ncbi.nlm.nih.gov/pmc/articles/PMC3410635/