A 22-year-old woman presents with recurrent jaundice, splenomegaly, and dark urine. Peripheral blood smear shows spherocytes. Direct Coombs test is negative. Osmotic fragility is increased. Her father has similar history. Which enzyme assay is MOST useful to confirm hereditary spherocytosis if clinical and laboratory features are classic but osmotic fragility is borderline?
- A G6PD enzyme assay
- B Eosin-5-maleimide (EMA) binding test by flow cytometry ✓
- C Acidified glycerol lysis test (AGLT)
- D Serum haptoglobin level
Explanation
The eosin-5-maleimide (EMA) binding test by flow cytometry is the most sensitive (93%) and specific (99%) diagnostic test for hereditary spherocytosis and is now the preferred screening test per current guidelines. EMA is a fluorescent dye that binds to RBC membrane proteins (band 3, Rh complex, CD47); reduced EMA fluorescence indicates deficiency of these proteins which occurs in HS due to spectrin/ankyrin/band 3 mutations. Osmotic fragility is less sensitive (especially in mild HS or after transfusion). G6PD assay diagnoses G6PD deficiency. AGLT has moderate sensitivity.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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