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Distinguishing features between hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and Southeast Asian ovalocytosis on the blood smear, ektacytometry, and EMA binding

Distinguishing features between hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and Southeast Asian ovalocytosis on the blood smear, ektacytometry, and EMA binding

RBCs from patients with HS, HE, and SAO have different properties that can be distinguished using specialized laboratory tests. Ektacytometry (Osmoscan) measures the deformability of the RBCs (on the Y-axis) across a gradient of osmolalities (on the X-axis). The minimal deformability index on the left of the scan is the osmolality where hemolysis occurs. EMA binding measures the band 3 content of RBCs, with the control in white and the patient samples in red (pink is the region of overlap).

(A) HS:

  • The blood smear shows spherocytes.
  • On ektacytometry, all measures of deformability (all parts of the tracing) are below the values for normal RBCs, consistent with a decrease in RBC minimal and maximal deformability. The minimal deformity is shifted to the right.
  • For EMA binding, the curve is shifted to the left due to reduced band 3 content.

(B) HE:

  • The blood smear shows elliptocytes.
  • On ektacytometry, the peak deformability (peak of the graph) is lower, consistent with lower maximal flexibility and decreased deformability under shear stress, but the outer edges of the peak (including Omin, the osmolality at which 50% of RBCs lyse) are relatively normal, consistent with less severe (or no) hemolysis in most cases.
  • For EMA binding, band 3 content is slightly increased in full elliptocytes and much lower in elliptocyte fragments from a severe HE patient.

(C) SAO:

  • The blood smear shows oval-shaped RBCs with 1 or 2 slits (also called theta cells or knizocytes)
  • On ektacytometry there is no discernable deformability across a wide osmotic gradient (characteristically nearly flat curve).
  • For EMA binding, the 8 amino acid deletion renders lysine 430 inaccessible to EMA, although total band 3 content is normal.
EMA: eosin-5-maleimide; HE: hereditary elliptocytosis; HS: hereditary spherocytosis; RBCs: red blood cells; SAO: Southeast Asian ovalocytosis.
Reproduced from: Zaidi AU, Buck S, Gadgeel M, et al. Clinical Diagnosis of Red Cell Membrane Disorders: Comparison of Osmotic Gradient Ektacytometry and Eosin Maleimide (EMA) Fluorescence Test for Red Cell Band 3 (AE1, SLC4A1) Content for Clinical Diagnosis. Front Physiol 2020; 11:636. Available at: https://www.frontiersin.org/articles/10.3389/fphys.2020.00636/full. Copyright © 2020 The Authors. Reproduced under the terms of the Creative Commons Attribution License 4.0.
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