Which test result would most strongly support hereditary spherocytosis over hereditary elliptocytosis?

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The key idea is how red cell membrane stability differs between these two inherited disorders. Hereditary spherocytosis arises from defects in proteins that anchor the membrane skeleton to the lipid bilayer, producing spherical red cells with a reduced surface area-to-volume ratio. Those spheres are fragile in hypotonic solutions, so they lyse more readily and the osmotic fragility test comes back positive. In hereditary elliptocytosis, the membrane skeleton defects create elongated cells, but the cells aren’t the same kind of fragile spheres in hypotonic saline, so osmotic fragility is typically normal. Therefore, a positive osmotic fragility test most strongly supports hereditary spherocytosis over hereditary elliptocytosis. Smear findings can help, with spherocytes suggesting HS and elliptocytes suggesting HE, but the osmotic fragility result directly reflects the membrane instability that distinguishes HS from HE. Echinocytes are not characteristic for these two conditions and don’t help differentiate them.