Proposed algorithm for laboratory workup of a patient presenting with hemolysis with or without anemia. In many cases of mild HS and most cases of PIEZO1-associated HX, anemia may be well compensated by reticulocytosis. Although our focus here is on RBC membrane disorders, the differential includes other causes of hemolytic disorders that are mentioned in this algorithm. Evaluation for autoimmune or, especially in an infant, alloimmune hemolytic anemia with DAT and IAT is the first testing recommended since such a diagnosis is frequently acute and evolving, requiring immediate action. Of note, warm autoimmune hemolytic anemia in children and occasionally adults with underlying immune dysregulation may be atypical and conventionally DAT-negative.^52-54  Consideration should also be given to the possibility of MAHA, PNH, and Wilson disease. The cases described in this review, especially that presented in Figure 6, demonstrate utilization of this algorithm. A blood smear review of the patient and parents and attention to the RBC indices including MCV, MCHC, and RDW, along with hemolytic markers (unconjugated bilirubin, lactate dehydrogenase, haptoglobin—of note, haptoglobin is reliable after 6 months of life since earlier it may be low due to decreased production by the infant's liver rather than increased consumption) and ferritin and transferrin saturation to consider iron-loading inefficient erythropoiesis, can provide hints as to the differential diagnosis. In a non-chronically transfused patient, we suggest phenotypic evaluation considering the differential of globin disorders, followed by RBC membranopathies and enzymopathies. Rare causes of HHA such as unstable Hgb disorders and CDAs also need to be considered. When suboptimal reticulocytosis or iron overload or skeletal abnormalities are noted in a patient with hemolytic anemia, the possibility of CDA should be considered and pursued. The combination of blood smear review and osmotic gradient ektacytometry frequently helps to narrow the differential while alerting clinicians of rare possibilities. Osmotic gradient ektacytometry evaluates the deformability of RBCs as they are subjected to constant shear stress in a medium of increasing osmolality in a laser diffraction viscometer and is the reference technique for differential diagnosis of erythrocyte membrane and hydration disorders when a recent transfusion does not interfere with phenotypic evaluation of the patient.⁴  Flow cytometry with eosin-5′-maleimide binding of band 3 and Rh-related proteins is a rapid screening test for RBC membrane disorders characterized by membrane loss.⁵⁵  Osmotic fragility is increased in HS and expected to be decreased in HX (however, it is reported as normal in patients with KCNN4 Arg352His mutation³¹ ). When the patient is recently or chronically transfused, as is typically the case for infants with HHA, genetic evaluation with clinically available NGS panels or research-based whole-exome sequencing or whole-genome sequencing may provide an accurate diagnosis necessary for appropriate management decisions. Laboratories that offer sequencing on genes or panels associated with HHAs and red cell membrane disorders can be found by searching the Genetic Testing Registry: https://www.ncbi.nlm.nih.gov/gtr/. e.g. https://www.ncbi.nlm.nih.gov/gtr/all/tests/?term=RBC%20membrane%20disorders. aHUS, atypical hemolytic uremic syndrome; HUS, hemolytic uremic syndrome; MAHA, microangiopathic hemolytic anemia; PNH, paroxysmal nocturnal hemoglobinuria; RDW, RBC distribution width; TTP, thrombotic thrombocytopenic purpura.