Evaluation of F cells in sickle cell disorders by flow cytometry – comparison with the Kleihauer–Betke's slide method

Adult F cell numbers are raised in inherited haemoglobin disorders, such as β‐thalassaemia and sickle cell anaemia, hereditary persistence of foetal haemoglobin, and some acquired conditions, such as juvenile myelomonocytic leukaemia, during acute erythropoietic stress and pregnancy. True foetal erythrocytes containing foetal amounts of HbF can also occur in the adult circulation during the leakage of HbF‐containing cells from the foetus to the maternal circulation. In normal adults, HbF is restricted to a small proportion (3–7%) of red blood cells (RBC), termed ‘F cells’. Techniques estimating the amount of HbF use lysates prepared from RBC, whereas those that estimate the adult F cell count use intact RBC. An accurate assessment of adult F cells in sickle cell disorders is important because increased adult F cells are associated with decreased morbidity in these disorders. In the present study, HbF levels were measured and adult F cell numbers were estimated in 100 blood samples (25 normal individuals, 25 sickle heterozygotes, 25 sickle homozygotes and 25 sickle β‐thalassaemia cases), using high pressure liquid chromatography for HbF levels, and flow cytometry and the Kleihauer–Betke (KB) acid elution microscope slide method for cell counts. Flow cytometry gave a more accurate assessment of adult F cells, eliminating any manual error, as compared to KB, which was less sensitive and precise as it is based on subjective visual interpretation.