eosin-5-maleimide

Eosin-5-maleimide is a fluorescent probe that covalently binds to lysine residues on transmembrane proteins of the red blood cell membrane, primarily band 3 (anion exchanger 1, AE1), as well as Rh-associated glycoprotein (RhAG) and Rh polypeptides. In the EMA-binding test, patient red blood cells are incubated with EMA dye, and the mean fluorescence intensity (MFI) is measured by flow cytometry and compared to normal controls.

Clinical significance: Hereditary spherocytosis is caused by mutations in genes encoding red cell membrane proteins—most commonly ankyrin, band 3 (SLC4A1), spectrin (alpha or beta), or protein 4.2. These defects destabilize the vertical linkages between the lipid bilayer and the underlying spectrin cytoskeleton, leading to progressive membrane loss, formation of spherocytes, and extravascular hemolysis (primarily in the spleen). Because band 3 is often deficient or dysfunctional in HS, EMA binding is reduced, resulting in decreased MFI on flow cytometry (typically >20% reduction compared to controls is suggestive of HS).

The EMA-binding test has largely replaced the osmotic fragility test as the preferred screening tool because it has higher sensitivity and specificity for HS, requires a smaller blood sample, and is less affected by coexisting conditions. However, it can have false positives/negatives in conditions like hereditary pyropoikilocytosis, Southeast Asian ovalocytosis, and congenital dyserythropoietic anemia type II, so results should be interpreted alongside clinical and laboratory findings (e.g., peripheral smear showing spherocytes, elevated MCHC, negative direct Coombs test to exclude autoimmune hemolytic anemia, family history).

Key boards association: Hereditary spherocytosis presents with hemolytic anemia, jaundice, splenomegaly, and increased risk of pigmented (calcium bilirubinate) gallstones. Treatment for severe cases is splenectomy, which resolves hemolysis but increases infection risk with encapsulated organisms (hence vaccination against Pneumococcus, Meningococcus, and H. influenzae prior to splenectomy).