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

Question

Accepted Answer

B. Osmotic fragility test result. ## 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.

Answer

A. Spherocytes on blood smear

Answer

C. Elevated indirect bilirubin

Answer

D. Positive direct antiglobulin test

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

Comparative Table

Pathophysiology

Why Osmotic Fragility Wins

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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