HS. Examples of red cell morphology in HS due to different gene mutations and associated osmotic gradient ektacytometry curves. Spherocytes (ie, RBCs with no or decreased central pallor) predominate, but additional RBC morphology characteristics may provide hints to the gene/protein defect causing HS, as Palek and Sahr³¹  and Eber and Lux¹³  have described very astutely in the past. (A-E, left) Blood smear of a patient with AD ANK1-HS showing anisocytosis (A). Occasional mushroom-shaped red cells (arrows) are characteristic of HS due to deficiency of band 3 (SLC4A1) (B). Acanthocytes (arrowheads) and echinocytes (arrow) are noted along with spherocytes in SPTB-associated HS (C). Blood smear of a not-recently transfused patient with AR SPTA1-HS demonstrates remarkable anisocytosis and poikilocytosis with contracted dense cells (D). Ovalocytes (arrows) and few ovalostomatocytes (arrowheads) are noted in AR EPB42-HS (E). Nevertheless, the above described changes in RBC morphology are not always specific for the gene affected in HS.¹⁰²  Scale bars, 14 µm; Wright-Giemsa stain. (A-E, right) The ektacytometry curve in HS is characterized by increased O_min, which corresponds to the increased osmotic fragility of the spherocytes. In almost all cases, decreased maximum deformability indicated by low EI_max is also noted, as well as decreased O_hyp. The decrease in EI_max tends to correlate with the degree of hemolysis and severity of anemia. (F) Important for differential diagnosis is that not all spherocytosis is hereditary. Autoimmune hemolytic anemia due to warm-reacting immunoglobulin G causes RBC membrane loss and acquired spherocytosis, with erythrocyte morphology and ektacytometry resembling HS. It is advisable to initiate the workup by first considering the possibility of an immune-mediated cause in every hemolytic anemia, since this radically alters management. In addition, blood smear and ektacytometry in congenital dyserythropoietic anemia type II also resemble HS; higher MCV, suboptimal reticulocytosis, and ferritin values disproportionally high for the number of transfusions will provide a clue to this possibility if bone marrow studies have not yet been performed. AIHA, autoimmune hemolytic anemia.