Immune thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by low platelet counts, typically in an otherwise healthy individual.^1,2 Sometimes referred to as idiopathic thrombocytopenia purpura, primary ITP has an unknown etiology.^1,3 

Epidemiology of ITP

Children

ITP prevalence among children is approximately equal to incidence (due to the short duration of most cases).⁴ European studies have reported that the annual incidence rate of ITP among children is approximately 4.3 to 5.3 cases per million.^5,6 A US study reported a prevalence of 72 cases per million for children; however, the authors conclude that this figure may be high due to the nonspecificity of the relevant ICD code.⁴ The incidence peaks at 5 years of age and young children usually experience ITP after an infectious illness.⁷ 
ITP affects boys and girls equally; for 70% to 80% of patients it resolves (with or without treatment) within 6 months.^7,8 In approximately 1 in 4 children ITP will have a chronic course⁵ and require standard treatment.

Adults

Adult ITP has an annual incidence of approximately 2 cases per 100,000. Estimates of the incidence of adult ITP are based primarily on studies from Denmark (2.7 per 100,000) and the United Kingdom (1.6 per 100,000).^6,9,10 Adult ITP is more likely than childhood ITP to take a chronic course. This is reflected in the age-adjusted US prevalence, estimated in a population-based Maryland study to be 9.5 cases per 100,000.⁴ There is no apparent prevalence difference between African Americans and whites.¹¹

ITP was historically considered a condition primarily affecting adult women 18 to 40 years of age.¹² However, today it is known to affect a wider range of individuals.

• The female-to-male ratio of ITP patients may be lower than previously thought, ranging from 1.2 to 1.9 women for each man.^4,9,10
• Female predominance was seen primarily in middle-aged adults and was not evident in patients older than 60 years of age.⁹
• Incidence rate appears to increase with age, with the highest age-specific incidence in patients older than 60 years of age.^10,12

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References

1. The American Heritage Stedman’s Medical Dictionary. Philadelphia, PA: Williams & Wilkins; 2001:401,404.
2. Chong BH, Ho S-J. Autoimmune thrombocytopenia. J Thromb Haemost. 2005;3(8):1763-1772.
3. George JN, Woolf SH, Raskob GE, et al. Idiopathic thrombocytopenic purpura: a practice guideline developed by explicit methods for the American Society of Hematology. Blood. 1996;88(1):3-40.
4. Segal JB, Powe NR. Prevalence of immune thrombocytopenia: analyses of administrative data. J Thromb Haemost. 2006;4(11):2377-2383.
5. Zeller B, Helgestad J, Hellebostad M, et al. Immune thrombocytopenic purpura in childhood in Norway: a prospective, population-based registration. Pediatr Hematol Oncol. 2000;17(7):551-558.
6. Fogarty PF, Segal JB. The epidemiology of immune thrombocytopenic purpura. Curr Opin Hematol. 2007;14: 515-519.
7. Cines DB, Blanchette VS. Immune thrombocytopenic purpura. N Engl J Med. 2002;346(13):995-1008.
8. British Committee for Standards in Haematology General Haematology Task Force. Guidelines for the investigation and management of idiopathic thrombocytopenic purpura in adults, children and in pregnancy. Br J Haematol. 2003;120(4):574-596.
9. Frederiksen H, Schmidt K. The incidence of idiopathic thrombocytopenic purpura in adults increases with age. Blood. 1999;94(3):909-913.
10. Neylon AJ, Saunders PWG, Howard MR, Proctor SJ, Taylor PRA. Clinically significant newly presenting autoimmune thrombocytopenic purpura in adults: a prospective study of a population-based cohort of 245 patients. Br J Haematol. 2003;122(6):966-974.
11. Landgren O, Gridley G, Fears TR, Caporaso N. Immune thrombocytopenic purpura does not exhibit a disparity in prevalence between African American and White veterans. Blood. 2006;108(3):1111-1112.
12. Cines DB, Bussel JB. How I treat idiopathic thrombocytopenic purpura (ITP). Blood. 2005;106(7):2244-2251.