A 32-year-old woman presents to the emergency department with spontaneous bleeding from gums and petechial rash over lower limbs for 3 days. She denies any recent infections or medication use. On examination, she has severe thrombocytopenia (platelet count 8,000/µL). Coagulation studies show: PT 12 sec (normal), aPTT 32 sec (normal), bleeding time prolonged. Bone marrow examination reveals adequate megakaryocytes. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of severe thrombocytopenia with normal coagulation studies (PT, aPTT) and adequate bone marrow megakaryocytes is pathognomonic for immune thrombocytopenia. ## Diagnostic Criteria for ITP | Feature | ITP | DIC | TTP | |---------|-----|-----|-----| | Platelet count | Severely ↓ | ↓ | ↓ | | PT/aPTT | Normal | Prolonged | Normal | | Fibrinogen | Normal | ↓ | Normal | | Schistocytes | Absent | Present | Present | | Bone marrow | Normal/↑ megakaryocytes | Normal | Normal | | Hemolytic anemia | Absent | Present | Present (MAHA) | ## Pathophysiology of ITP 1. **Autoimmune mechanism**: IgG antibodies against platelet surface antigens (GPIIb/IIIa, GPIb/IX) 2. **Platelet destruction**: Antibody-coated platelets are sequestered and destroyed in the spleen 3. **Bone marrow response**: Increased megakaryopoiesis (adequate/increased megakaryocytes on marrow exam) 4. **Coagulation cascade**: Remains intact → normal PT, aPTT, fibrinogen **High-Yield:** In ITP, the **only** abnormality is thrombocytopenia; all other hemostatic parameters are normal. This distinguishes it from DIC (where coagulation factors are consumed) and TTP (where microangiopathic hemolytic anemia dominates). **Clinical Pearl:** Prolonged bleeding time in ITP reflects the severe reduction in platelet count, not a platelet function defect. Modern labs have largely abandoned bleeding time testing in favor of platelet count and peripheral smear. ## Why This Patient Has ITP - **Acute presentation** (3 days) with severe thrombocytopenia - **No systemic features** (no fever, no neurological signs, no renal dysfunction) → rules out TTP - **Normal coagulation studies** → rules out DIC (which shows prolonged PT/aPTT, low fibrinogen, elevated D-dimer) - **Adequate megakaryocytes** → confirms peripheral destruction, not bone marrow failure - **No recent infection or drug** → less likely secondary ITP **Mnemonic: PLATE** — Platelets low, Liver/Spleen enlarged (sometimes), Antibodies against platelets, Thrombocytopenia immune-mediated, Excellent bone marrow response.