A 28-year-old woman presents to the emergency department with a 3-day history of spontaneous bruising over her lower limbs, petechiae on her forearms, and a single episode of epistaxis. She denies any recent infections, medications, or family history of bleeding disorders. Vital signs are stable. Physical examination reveals scattered petechiae and purpura but no hepatosplenomegaly or lymphadenopathy. Laboratory investigations show: Hemoglobin 12.8 g/dL, WBC 7,200/μL, Platelet count 8,000/μL, PT 12 s (normal), aPTT 32 s (normal), Bleeding time 8 min (prolonged). Bone marrow examination shows normal megakaryocytes with increased numbers. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Immune Thrombocytopenia (ITP) ### Key Clinical Features **Key Point:** ITP is characterized by isolated thrombocytopenia with normal or increased bone marrow megakaryocytes, normal coagulation studies, and a clinical presentation of mucocutaneous bleeding without systemic features. This patient presents with: - Severe thrombocytopenia (8,000/μL) with mucocutaneous bleeding (petechiae, purpura, epistaxis) - **Normal PT and aPTT** — rules out coagulopathy - **Normal hemoglobin and WBC** — excludes hemolytic or systemic process - **Increased megakaryocytes on bone marrow** — indicates peripheral destruction, not bone marrow failure - **Absence of systemic features** — no fever, hemolysis, renal dysfunction, or neurological signs ### Pathophysiology ITP results from autoimmune destruction of platelets by IgG antibodies (anti-platelet antibodies) binding to platelet surface antigens, leading to Fc-receptor-mediated clearance in the spleen. The bone marrow responds by increasing megakaryocyte production, but destruction exceeds production. ### Diagnostic Criteria | Feature | ITP | TTP | DIC | |---------|-----|-----|-----| | **Platelet count** | <30,000/μL | <30,000/μL | <100,000/μL | | **PT/aPTT** | Normal | Normal | Prolonged | | **Fibrinogen** | Normal | Normal | Low | | **Schistocytes** | Absent | Present | Present | | **Fever** | Absent | Present | Variable | | **Renal dysfunction** | Absent | Present | Present | | **Bone marrow** | Increased MKs | Normal | Normal | ### Why This Is ITP **High-Yield:** The combination of **isolated thrombocytopenia + normal coagulation studies + increased bone marrow megakaryocytes + mucocutaneous bleeding** is pathognomonic for ITP. **Clinical Pearl:** Bleeding time is prolonged in ITP due to severe thrombocytopenia, but this is a non-specific finding. The critical diagnostic clue is the **normal PT/aPTT** (ruling out coagulopathy) and **increased megakaryocytes** (ruling out bone marrow failure). ### Management Approach ```mermaid flowchart TD A[Platelet count < 30,000/μL + Mucocutaneous bleeding]:::outcome --> B{Acute or Chronic?}:::decision B -->|Acute presentation| C[Corticosteroids first-line]:::action B -->|Chronic| D[Assess bleeding risk]:::decision D -->|Symptomatic| E[Corticosteroids or IVIG]:::action D -->|Asymptomatic| F[Observation]:::action C --> G{Response?}:::decision G -->|Good| H[Taper steroids]:::action G -->|Poor| I[IVIG or Splenectomy]:::action ``` **Mnemonic for ITP diagnosis: IMMUNE** - **I**solated thrombocytopenia - **M**egakaryocytes increased - **M**ucocutaneous bleeding - **U**nder 30,000 platelets - **N**ormal coagulation studies - **E**xclude other causes [cite:Harrison 21e Ch 139]