Immune thrombocytopenia (ITP) is a rare autoimmune disorder that causes the body’s immune system to mistakenly attack and destroy its own platelets, which are the blood cells responsible for clotting. The condition leads to a low platelet count, which can increase the risk of bleeding and bruising. Because ITP is uncommon and often challenging to diagnose definitively, calculating precisely how many people are affected remains difficult. Researchers rely on two primary statistics: incidence, which tracks new cases, and prevalence, which tracks the total number of people living with the condition at any given time.
Understanding Immune Thrombocytopenia
Immune thrombocytopenia is characterized by a significant decrease in the number of circulating platelets in the blood, a condition known as thrombocytopenia. The formal diagnostic threshold established by international consensus is a platelet count below 100,000 per microliter of blood, compared to a normal range that typically starts at 150,000 per microliter. The underlying mechanism involves the production of autoantibodies, usually immunoglobulin G (IgG), that bind to specific proteins on the surface of the platelets.
These antibody-coated platelets are then recognized as foreign by specialized immune cells, primarily in the spleen, which leads to their premature destruction and removal from the circulation. Furthermore, the immune response can also impair the ability of bone marrow cells, called megakaryocytes, to produce new platelets efficiently. The combination of increased platelet destruction and often inadequate production results in the low counts that define the disorder.
Tracking New Cases: Incidence Rates
Incidence measures the rate of new ITP diagnoses occurring each year within a specific population. Recent studies suggest the overall annual incidence in the United States is approximately 6.1 new cases for every 100,000 people. This figure is notably higher than older estimates, which typically placed the rate between 2.5 and 3.9 per 100,000 persons.
The rate of new diagnoses varies significantly across different age groups, showing a bimodal distribution. The highest incidence rates are observed in two distinct populations: very young children and older adults. For children aged zero to four years old, the incidence can be as high as 8.1 per 100,000 children annually.
In adults, the rate generally increases with age, peaking dramatically in those 65 years and older, where the incidence reaches an estimated 13.7 new cases per 100,000 persons each year. While the overall adult incidence is often cited in the range of 1.6 to 3.9 per 100,000 person-years, a more recent, refined estimate suggests about 3.3 new cases per 100,000 adults annually. The incidence also shows a slight difference between sexes, with studies indicating a higher rate in females compared to males overall.
The Total Count: Prevalence Across Populations
Prevalence refers to the total number of people currently living with ITP, including both newly diagnosed and long-term cases. Estimates of total ITP prevalence in the United States generally fall around 9.5 people per 100,000. Translating this rate to a total number of patients is difficult due to variations in defining the chronic versus acute nature of the disease.
One analysis of adult chronic ITP—defined as the condition persisting for longer than 12 months—estimated a prevalence of 23.6 per 100,000 adults. This specific estimate corresponds to approximately 52,700 adult chronic ITP cases in the United States. Like incidence, the prevalence of ITP increases with age, rising from about 17.0 per 100,000 in adults aged 18 to 49 to 36.2 per 100,000 in those 65 and older. Globally, it is estimated that over 200,000 people are living with ITP at any given time.
Why Exact Numbers Remain Elusive
Establishing a single, precise number for ITP is challenging because it is a diagnosis of exclusion. Since no single laboratory test confirms ITP, physicians must first rule out all other potential causes of low platelet counts. Misdiagnosis of secondary ITP (thrombocytopenia caused by an underlying condition like HIV or Lupus) as primary ITP can occur, skewing the numbers.
Clinical Course and Underreporting
Another complicating factor is the clinical course of the disease, which is categorized by duration: newly diagnosed (less than three months), persistent (three to twelve months), and chronic (more than twelve months). Many cases of ITP, especially in children, are acute and resolve on their own, meaning they are counted in incidence but disappear from prevalence statistics after a year. Furthermore, mild cases can be asymptomatic, leading to underdiagnosis and underreporting. Statistical studies that rely on medical claims databases also face limitations because the medical coding used for ITP is not always exclusive to the condition, introducing potential inaccuracies in case identification.