A 38-year-old woman with chronic hemolytic anemia has a peripheral blood smear showing polychromasia, nucleated RBCs, and spherocytes. Which finding on the smear best distinguishes hereditary spherocytosis from autoimmune hemolytic anemia?

Explanation

## Distinguishing Hereditary Spherocytosis from Autoimmune Hemolytic Anemia ### Key Discriminating Features **Key Point:** While both hereditary spherocytosis (HS) and autoimmune hemolytic anemia (AIHA) present with spherocytes on blood smear, the **direct antiglobulin test (DAT/Coombs)** is the gold standard discriminator. HS is DAT-negative, whereas AIHA is DAT-positive. Additionally, HS has a genetic basis (family history) while AIHA is acquired. ### Comparison Table | Feature | Hereditary Spherocytosis | Autoimmune Hemolytic Anemia | |---------|--------------------------|------------------------------| | **DAT (Coombs)** | **Negative** | **Positive** | | **Spherocytes** | Present | Present | | **Osmotic Fragility** | **Increased** | Normal | | **Family History** | Often positive | Absent | | **Etiology** | Genetic (membrane defect) | Acquired (autoantibodies) | | **Polychromasia** | Present (reticulocytosis) | Present (reticulocytosis) | | **Nucleated RBCs** | Present (severe cases) | Present (severe cases) | | **Agglutination** | Absent on saline | May be present (cold AIHA) | ### High-Yield Discriminators **High-Yield:** The **DAT/Coombs test is the single best discriminator** between HS and AIHA. HS is intrinsically a membrane defect with no immune component (DAT-negative), while AIHA involves IgG or IgM antibody coating RBCs (DAT-positive). **Clinical Pearl:** HS patients have **osmotic fragility** — their spherical RBCs lyse in hypotonic saline at higher osmotic pressures than normal RBCs. AIHA RBCs have normal osmotic fragility. This osmotic fragility test, combined with negative DAT, confirms HS. **Mnemonic:** **HS = Hereditary, Spheres, Saline-fragile, Negative DAT** vs. **AIHA = Acquired, Antibodies, Agglutination, Positive DAT** ### Pathophysiology ### Hereditary Spherocytosis 1. Genetic defect in RBC membrane proteins (spectrin, ankyrin, band 3, protein 4.2) 2. Loss of biconcave shape → spherical RBCs 3. Increased osmotic fragility 4. Splenic trapping and hemolysis 5. **No immune component → DAT negative** ### Autoimmune Hemolytic Anemia 1. Autoantibodies (IgG warm, IgM cold) against RBC antigens 2. Complement activation and/or opsonization 3. Splenic and hepatic destruction 4. **Antibodies coat RBCs → DAT positive** **Warning:** Both conditions present with spherocytes, polychromasia, and reticulocytosis. Do NOT assume HS based on morphology alone — the DAT is mandatory to distinguish them. ### Clinical Approach ```mermaid flowchart TD A[Hemolytic Anemia with Spherocytes]:::outcome --> B{DAT/Coombs Test}:::decision B -->|Positive| C[Autoimmune Hemolytic Anemia]:::outcome C --> D[Warm or Cold AIHA]:::action B -->|Negative| E{Osmotic Fragility Test}:::decision E -->|Increased| F[Hereditary Spherocytosis]:::outcome E -->|Normal| G[Other Causes: G6PD, Thalassemia, etc.]:::outcome F --> H[Confirm with Family History & Genetic Testing]:::action ``` [cite:Robbins 10e Ch 13]