A 28-year-old woman presents with jaundice, dark urine, and pallor. Laboratory investigations reveal hemoglobin 8.2 g/dL, reticulocyte count 12%, elevated indirect bilirubin, elevated LDH, and low haptoglobin. Flow cytometry shows absence of CD55 and CD59 on blood cells. What is the drug of choice for acute hemolytic episodes in this patient?

Explanation

## Clinical Diagnosis The clinical and laboratory findings are diagnostic of **Paroxysmal Nocturnal Hemoglobinuria (PNH)**: - Hemolysis with elevated reticulocyte count, indirect bilirubin, and LDH - Low haptoglobin (consumed by free hemoglobin) - **Flow cytometry showing absence of CD55 and CD59** — these are GPI-anchored complement regulatory proteins ## Drug of Choice: Eculizumab **Key Point:** Eculizumab is a **C5 complement inhibitor** that blocks the terminal complement cascade, preventing formation of the membrane attack complex (MAC) that causes intravascular hemolysis in PNH. **High-Yield:** PNH is the only hemolytic anemia where complement inhibition is the primary therapeutic target. Eculizumab is the gold standard and FDA-approved for PNH. **Clinical Pearl:** Patients on eculizumab require meningococcal vaccination (serogroups A, C, W, Y, and B) due to increased risk of Neisseria meningitidis infection when C5 is blocked. ## Mechanism of Hemolysis in PNH ```mermaid flowchart TD A[Defective GPI-anchored proteins<br/>CD55 and CD59 absent]:::outcome --> B[Loss of complement regulation] B --> C[Uncontrolled C3 and C5 activation] C --> D[Formation of MAC<br/>C5b-C9 complex]:::urgent D --> E[Intravascular hemolysis] F[Eculizumab<br/>C5 inhibitor]:::action --> G[Blocks MAC formation] G --> H[Prevents hemolysis]:::outcome ``` ## Why Eculizumab Over Alternatives | Feature | Eculizumab | Corticosteroids | Rituximab | |---------|-----------|-----------------|----------| | Mechanism | C5 complement inhibitor | Immunosuppression | B-cell depletion | | Efficacy in PNH | Excellent (blocks MAC) | Poor (not effective) | Minimal role | | Evidence | FDA-approved, gold standard | No proven benefit | Rarely used | | Indication | First-line for hemolysis | Not recommended | Refractory cases only | **Note:** Splenectomy is not an option for PNH because hemolysis is intravascular (complement-mediated), not extravascular; the spleen does not remove complement-coated cells efficiently.