Fig. 3.

Single-RBC saturation distributions reveal large differences in RBC subpopulations despite modest differences in mean saturations. A shows mean single-RBC hemoglobin saturation as a function of oxygen (dots in Left) and single-RBC saturation histograms for a healthy HbA sample (H) at 6 different oxygen tensions. B shows the same plots for a sickle cell sample (SS1) with 74% HbS, 21% HbF, and 5% HbA2. The mean saturations as function of oxygen tension are only modestly shifted downward relative to the dots in A, Left for sample H. The shapes of the histograms in B are markedly different from those in A at the intermediate oxygen tensions, with 2 modes instead of 1. The dots in Left show means of the upper and lower peaks of the saturation distributions at each measured oxygen tension. The saturations in the upper peaks are generally consistent with those in sample H, while the saturations in the lower peak are intermediate between those of sample H and what is expected for pure hemoglobin polymer as shown by the blue curve in the leftmost panel. (See Fig. 4 for more detail on fits used to define upper and lower peaks.) C shows results for another sickle cell sample (SS2) with 82% HbS, 14% HbF, and 4% HbA2. The dots in the leftmost panel show means of the upper and lower peaks of the saturation distributions at each measured oxygen tension as in B. At intermediate oxygen tensions (4.5, 5.9, and 7.3%) both SCD samples show a bimodal oxygen saturation distribution as described above and in Fig. 2, but the relative sizes of the 2 populations vary with oxygen tension and vary between patients. The curves in Left in each row also show the typical hemoglobin-oxygen dissociation curve for monomeric (red) and polymeric (blue) Hb.