Which finding best differentiates hereditary spherocytosis from autoimmune hemolytic anemia in a 28-year-old woman presenting with jaundice and splenomegaly?

Explanation

## Distinguishing Hereditary Spherocytosis from Autoimmune Hemolytic Anemia ### Key Discriminating Test **Key Point:** The osmotic fragility test is the single best discriminator between hereditary spherocytosis (HS) and autoimmune hemolytic anemia (AIHA). HS shows **increased osmotic fragility** (lysis at higher osmotic pressures), while AIHA shows **normal osmotic fragility**. ### Comparative Table | Feature | Hereditary Spherocytosis | Autoimmune Hemolytic Anemia | | --- | --- | --- | | **Osmotic Fragility** | **Increased (abnormal)** | **Normal** | | Direct Antiglobulin Test (DAT/Coombs) | Negative or weakly positive | Strongly positive | | Spherocytes on smear | Present (primary defect) | Present (secondary to hemolysis) | | Indirect Bilirubin | Elevated | Elevated | | Reticulocyte Count | Elevated | Elevated | | Family History | Often positive | Absent | | Splenectomy Response | Curative | Temporary benefit | ### Pathophysiology **High-Yield:** Osmotic fragility test exploits the structural defect in HS: - **HS:** Membrane protein defects (spectrin, ankyrin, band 3) cause loss of biconcave shape → spherocytes are rigid and lyse at higher osmotic pressures (lower NaCl concentration). - **AIHA:** RBCs are structurally normal; antibody coating causes hemolysis, but osmotic fragility remains normal. ### Why Osmotic Fragility Wins 1. **DAT/Coombs test** is positive in AIHA but negative in HS—however, HS can have weakly positive DAT, making this less reliable. 2. **Spherocytes on smear** appear in both conditions; in AIHA, they form secondary to antibody-mediated damage. 3. **Elevated indirect bilirubin** occurs in both hemolytic anemias. 4. **Osmotic fragility** directly tests the membrane defect unique to HS. **Clinical Pearl:** HS osmotic fragility is **increased** (lysis at 0.36–0.4% NaCl), whereas normal RBCs lyse at 0.3% NaCl. AIHA RBCs lyse at normal osmotic pressure. **Mnemonic:** **HS = Hereditary Spherocytes = Higher fragility** — Remember: HS spherocytes are fragile in hypotonic solutions.