RBCs from ME/CFS exhibited reduced capillary velocity and sensitivity to reduced PO₂. (A) Schematic of the microfluidic setup to measure ex vivo RBC capillary velocity at varied PO₂ levels (top). Representative time-lapse images of RBCs from HCs and patients with ME/CFS flowing through a microfluidic capillary at PO₂ of 0 mm Hg (bottom; scale bar, 25 μm). (B) The magnitude of capillary velocity and calculated velocity slope (ie, sensitivity) of RBCs from ME/CFS are significantly reduced compared to HCs (HCs, R² = 0.99; n = 23 participants; 2556 cells; ME/CFS, R² = 0.92; n = 35 patients; 3847 cells). ns, P > .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001 (Mann-Whitney test). Unpaired t test was used for slope comparison. Each data point in the calculated slope represents the slope from 1 participant. (C) Calculated slope and average RBC velocity (average of 4 PO₂) showed no significant dependence on participant age (HCs, n = 23; ME/CFS, n = 35). Each data point represents 1 participant. (D-E) RBC capillary velocity but not the calculated slope (ie, sensitivity) was significantly different between sexes for HCs (D) and ME/CFS (E). Nine healthy male (R² = 0.93) and 14 healthy female participants (R² = 1.00) and 10 male (R² = 0.48) and 25 female (R² = 0.93) patients with ME/CFS were included. Each data point in the calculated slope represents 1 participant. Data are presented as mean ± standard error of the mean (SEM).