Immune thrombocytopenia (ITP) is an autoimmune disease but, paradoxically, and unlike other autoimmune diseases, antiplatelet antibodies are not used either for the diagnosis of the disease or for its prognosis. ITP is a diagnosis of exclusion retained after elimination of other pathologies leading to a thrombocytopenia. No major study has prospectively evaluated the diagnostic value of the presence of anti-platelet antibodies in the etiological investigation of a thrombocytopenia, nor the impact of platelet antibodies on the course of ITP. The gold standard analysis for the determination of platelet antibodies, is the "monoclonal antibody immobilization of platelet antigens" assay (MAIPA), either direct to detect autoantibodies attached to platelets, or indirect to detect circulating antiplatelet antibodies. Therefore, this work aims to study the contribution of the presence of anti-platelet antibodies detected in MAIPA to determine the autoimmune nature of a thrombocytopenia at diagnosis.

Inclusion Criteria:

• Patient over 18 years old;
• Patients with thrombocytopenia <100 G/L, checked twice, having ruled out false thrombocytopenia by platelet aggregation and acute leukemia by smear;
• No treatment started;
• Free, informed and written consent signed by the participant and the investigator (no later than the day of inclusion and prior to any review required by the research);
• Person affiliated or benefiting from a social security scheme.

Exclusion Criteria:

• Secondary ITP;
• False thrombocytopenia;
• Patients who have been transfused with platelets for less than 7 days with efficacy;
• Patient treated for thrombocytopenia (48 hours of corticosteroid therapy is tolerated and is not an exclusion criteria);
• Patient with acute leukemia;
• Pregnant or breastfeeding woman;
• False thrombocytopenia;
• Patient under guardianship, curatorship or any other legal protection regime.