Hypothetical distribution of HbF/F-cell and the percent F-cells in 3 patients with HbF levels of 5%, 10%, and 20%. (A) HbF 5%. (B) HbF 10%. (C) HbF 20%. Plots were generated by simulating a sample of 1000 cells in each plot using a β distribution defined in the range 0 to 30, with mean HbF content per cell of 5% (μ = 1.5 pg), 10% (μ = 3 pg), and 20% (μ = 6 pg). With a fixed mean, the standard deviation (SDev) was changed to show how the distribution of HbF per cell can greatly vary even if the mean is the same (see text for additional details). The simulations were conducted using the R package.²¹  Vertical dashed lines show the approximate lower level of HbF/F cells needed to detect F-cells by fluorescence-activated cell sorter, and the solid vertical line shows the approximate lower level of HbF needed to protect the cell from deoxyHbS polymerization at physiologic O₂ saturations.