Estimating prevalence is essential to understand the population burden of a disorder. However estimating the prevalence of ITP is challenging because there are no specific diagnostic criteria that document the presence of ITP; the diagnosis can only be established by excluding other causes of thrombocytopenia.(1
) In addition, ITP is uncommon(15
) and therefore potentially unfamiliar to both physicians and medical record coders.
To determine the prevalence of ITP regardless of age, clinical characteristics, insurance status, or source of health care, we surveyed hematologists in the State of Oklahoma to determine the number of patients identified as outpatients utilizing the administrative code ICD-9-CM 287.3. This method was based on three assumptions:  that the administrative code ICD-9-CM 287.3 was sufficiently accurate to identify both children and adults with ITP,  that most patients with ITP were diagnosed and/or managed by hematologists, and  that most patients with ITP were diagnosed and/or managed as outpatients. The validity of these three assumptions was established by our previous review of OUMC medical records in which we established a definition for a definite diagnosis of ITP.(19
) Data from this prior study documented that the PPV of the administrative code ICD-9-CM 287.3 for identifying patients with a definite diagnosis of ITP by a hematologist was 0.72 for children and 0.69 for adults, similar to a previous report on the accuracy of this administrative code for identification of patients with ITP which determined a PPV of 0.71.(23
) We also documented that in 95% of children and 92% of adults with a definite diagnosis of ITP, the diagnosis was established by a hematologist; this supported the validity of including only hematologists and not all physicians for our determination of prevalence. Finally, we determined that in 95% of children and 83% of adults with a definite diagnosis of ITP established by a hematologist, the diagnosis was established as an outpatient; this supported the validity of surveying only the offices of hematologists and not records of all hospitals. Therefore these data documented that a survey of hematologists’ offices for patients with ITP identified by the administrative code ICD-9-CM 287.3 was an appropriate method to determine the prevalence of patients with ITP. Because we were successful in obtaining data from 88 (95%) of the state’s 93 hematologists, this was also a feasible method to determine the prevalence of ITP in Oklahoma.
Adjustment of the average annual prevalence estimates for all three of the assumptions in our method resulted in lower average annual prevalence estimates for both children (7.4 per 100,000 children) and adults (11.1 per 100,000 adults) but both adjusted estimates were within the 95% confidence intervals of our observed estimates. Therefore adjustment for these three assumptions did not significantly impact the magnitude of the observed prevalence estimates.
In children, ITP is typically an acute and spontaneously resolving disorder. ITP resolves within six months of initial diagnosis in 70% of children(20
) and within 12 months in nearly 80% of children.(11
) Consistent with the uncommon persistence of ITP, the annual incidence of chronic ITP in children, defined as persistence for more than 6 months after diagnosis, has been reported to be 0.46 per 100,000 children,(24
) while the annual incidence of all (acute and chronic) children with ITP is between 1.9 and 6.4 per 100,000 children.(15
) Since the clinical course of ITP in most children is less than one year, it may be expected that the annual prevalence of ITP in children would be similar to the annual incidence. However the average annual prevalence for ITP in children, estimated from our data, 8.1 per 100,000 children, is higher than the reported annual incidence.(15
) One possible explanation for the higher prevalence may be continued follow-up of children after the ITP resolves. The prevalence of chronic ITP in children has been reported to be 4.6 per 100,000 children,(16
) approximately half of our observed prevalence for all (acute and chronic) children with ITP. Our average annual prevalence estimate for children of 8.1 per 100,000 children (95% CI 6.7, 9.5) is not appreciably different from the previously reported annual prevalence estimate for children of 7.2 per 100,000 children aged 1–14 years.(17
In adults, ITP is typically a chronic disorder, commonly persistent for many months or years unless effectively treated by splenectomy or immunosuppressive agents.(1
) Therefore it would be expected that the prevalence would be greater than the incidence. Our average annual prevalence estimate for adults of 12.1 per 100,000 adults (95% CI 11.1, 13.0), is nearly four-fold greater than the incidence of ITP in adults, 3.3 per 100,000 adults per year.(15
Segal and Powe(17
) previously reported the annual prevalence of ITP for privately insured patients ages 1–64 years to be 9.6 (95% CI, 8.6, 11.0) per 100,000 persons (). Feudjo-Tepie, et al.(14
) previously reported the annual prevalence of chronic ITP in privately insured adults ages 18–64 years to be 7.1 for 2002 and 9.5 for 2004 per 100,000 adults. Using data from the United Kingdom General Practice Research Database, Bennett, et al.(18
) previously reported the annual prevalence for adults 18 years old and older for 2003 to be 4.0 per 100,000 adults and for 2004 to be 4.6 per 100,000 adults. The annual prevalence of Segal and Powe(17
) is lower than our estimate of the prevalence for children and adults and the annual prevalence of Feudjo-Tepie, et al.(14
) is lower than our estimate of the prevalence for adults. Because of differences in inclusion criteria these estimates are not directly comparable. However, our estimate of prevalence was higher probably because we included patients over 64 years old and included patients without private insurance. Our annual prevalence estimates are higher than the adult estimates from Bennett, et al. (18
) possibly due to the differences between cases identified from administrative data compared to electronic medical records or differences between the USA and UK.
Our prevalence data for children are consistent with previous reports that the annual incidence of ITP in children is greater among children younger than age 10 years than among children 10 years and older.(20
) The predominance of women among patients age 20–39 years is similar to the increased relative frequency of other autoimmune disorders such as systemic lupus erythematosus in young women.(26
) The progressively increasing prevalence among older people and a greater prevalence of men relative to women among people 60 years and older is notable. Segal and Powe(17
) and Bennett, et al.(18
) also described increased prevalence among older adults. The increased prevalence of ITP among older people is consistent with previous reports that the annual incidence of ITP is greatest among the oldest age cohorts;(5
) in one of these reports the increased incidence among older males was also described.(13
A limitation of this study was that not all hematology offices across the state were able to provide data for the entire time 10-year time period. However estimates derived for different time periods maximized the use of the available data. The two-year prevalence estimates represented 94% of the state’s hematologists. However, the two-year estimate is a conservative estimate of prevalence because some ITP patients may not visit a hematologist within a two-year period. This study also required diagnosis of ITP by a hematologist, a requirement that could limit the comparability of our results to previous studies. However the requirement for diagnosis by a hematologist reflects the current referral practice in Oklahoma. A survey of primary care providers in the Oklahoma Practice-based Research Network documented that 75% of respondents were ‘likely’ to send a patient with moderate thrombocytopenia (platelet count 30,000/μL) to a hematologist for further evaluation and management. The likelihood of referral increased to 85% when the moderate thrombocytopenia was associated with mild bleeding symptoms (petechiae) and to 92% when the patient presented with severe thrombocytopenia (platelet count 10,000/μL).(28
) Some hematology offices may focus more on oncology rather than benign hematology which could raise concern in potential differences in diagnostic coding between groups.
In summary, our prevalence estimates are generalizable beyond privately insured persons and not limited by age. The generalizability of our data is supported by the similarity of our estimates to estimates previously reported from Maryland.(17
) This study also provides the best current estimates of the age and gender distribution of ITP throughout the population. These data will be important to assess the population burden of ITP.