## Diagnostic Approach to Immune Thrombocytopenia (ITP) **Key Point:** ITP is a diagnosis of exclusion based on clinical and laboratory findings; antiplatelet antibody detection provides supportive evidence of immune-mediated platelet destruction. ### Why Antiplatelet Antibody Detection (PAIgG/PAIgM) is Correct Antiplatelet antibody assays (PAIgG and PAIgM) detect immunoglobulin bound to platelet surfaces, indicating immune-mediated destruction. While not 100% sensitive or specific, they provide the most direct evidence of autoimmune platelet destruction in the context of: - Thrombocytopenia with adequate bone marrow megakaryocytes - Clinical features of bleeding (bruising, epistaxis, menorrhagia) - Absence of secondary causes **High-Yield:** PAIgG is more commonly elevated in ITP than PAIgM; combined testing increases sensitivity to ~60–70%. ### Diagnostic Criteria for ITP | Finding | Expected in ITP | |---------|----------------| | Platelet count | <150,000/μL | | Bone marrow megakaryocytes | Normal or increased | | Antiplatelet antibodies (PAIgG/PAIgM) | Positive (60–70%) | | Secondary causes | Absent | | Splenomegaly | Absent (if present, reconsider diagnosis) | **Clinical Pearl:** The absence of splenomegaly and the presence of adequate megakaryocytes on bone marrow examination help exclude other causes of thrombocytopenia (e.g., hypersplenism, bone marrow failure, TTP, DIC). ### Why Other Investigations Are Secondary - **Flow cytometry for platelet-associated immunoglobulin:** More sensitive than PAIgG/PAIgM assays but not routinely used for diagnosis; reserved for research or when conventional tests are equivocal. - **Bone marrow biopsy with immunohistochemistry:** Already performed; confirms adequate megakaryocytes but does not directly assess immune destruction. - **Platelet aggregation studies:** Used to diagnose platelet function disorders (e.g., Glanzmann thrombasthenia), not immune thrombocytopenia.
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