The incidence of idiopathic thrombocytopenic purpura in adults increases with age.

With the aim of determining the incidence of idiopathic thrombocytopenic purpura (ITP) in adults, we searched all adult ITP patients diagnosed from April 1, 1973 to December 31, 1995 in the County of Funen in Denmark. This county comprises 9% of the total Danish adult population. A total of 221 patients fulfilled the inclusion criteria, yielding an annual standardized incidence rate of 2.68 per 100,000. The median age of the patient population was 56 years, and the female to male ratio was 1.7. Changing the platelet count cut-off point from 100 x 10(9)/L to 50 x 10(9)/L changed the incidence rate to 2.25 per 100,000. Comparing patients less and more than 60 years old, the incidence rate more than doubled and the sex difference was eliminated in the older age group. These two age groups were almost identical regarding platelet count at diagnosis and number of asymptomatic cases. The incidence rate increased in the study period. This increase in particular involved asymptomatic patients and old males who were both symptomatic or not symptomatic. Including additional patients identified by a questionnaire study of the contribution from the primary care physicians and the practicing specialists in the second half of the study period, a reliable estimate of the annual ITP incidence in Danish adults, using a platelet concentration cut-off point of 50 x 10(9)/L, is 3.2 per 100, 000 persons.