Which morphological finding on peripheral blood smear is pathognomonic for hereditary spherocytosis?

Explanation

## Hereditary Spherocytosis: Peripheral Blood Smear Findings **Key Point:** Spherocytes are the hallmark morphological finding in hereditary spherocytosis (HS), characterized by dense, small, dark-staining RBCs lacking central pallor. ### Morphological Features of Spherocytes | Feature | Spherocyte | Normal RBC | |---------|-----------|----------| | Shape | Spherical, dense | Biconcave disc | | Central pallor | Absent or reduced | Present (1/3 diameter) | | Staining intensity | Darker (increased Hb concentration) | Lighter | | Osmotic fragility | Increased (lyse at higher saline concentrations) | Normal | **High-Yield:** Osmotic fragility test is the confirmatory test for HS — spherocytes lyse at higher saline concentrations (0.36% vs. normal 0.30%) because they have reduced surface area-to-volume ratio and cannot accommodate additional water influx. ### Why Spherocytes Form in HS 1. Defective membrane proteins (spectrin, ankyrin, band 3, protein 4.2) 2. Loss of membrane lipids and surface area 3. Progressive loss of biconcave shape → spherical form 4. Increased osmotic fragility and mechanical fragility 5. Splenic sequestration and hemolysis **Clinical Pearl:** Spherocytes are not pathognomonic for HS alone — they also appear in autoimmune hemolytic anemia (AIHA), but the osmotic fragility test distinguishes HS (increased fragility) from AIHA (normal fragility). **Mnemonic:** **SPHERO** — **S**pectrin defects, **P**rotein 4.2 loss, **H**ereditary, **E**rythrocyte membrane, **R**educed surface, **O**smotic fragility increased. [cite:Robbins 10e Ch 12]