Peripheral blood T-cell activation status readily distinguishes patients with HLH and sepsis. (A-B) Representative flow cytometric plots and cumulative data comparing the frequency of CD38^high or CD38^high/HLA-DR⁺ (double-positive) CD8⁺ T cells in pediatric controls (C), patients with sepsis (S), or patients with HLH (H). (C) ROC analysis of CD38^high/HLA-DR⁺ (double-positive) CD8⁺ T cells, comparing HLH and sepsis. (D-E) Representative FACS plots and cumulative data comparing the frequency of CD38^high or CD38^high/HLA-DR⁺ (double-positive) CD4⁺ T cells in pediatric controls, patients with sepsis, or patients with HLH. (F) ROC analysis of CD38^high/HLA-DR⁺ (double-positive) CD4⁺ T cells, comparing HLH and sepsis. Data are representative of 27 pediatric controls, 9 to 19 patients with sepsis and 19 to 43 patients with HLH. Error bars represent median with 95% CI. Differences between indicated groups were calculated using the unpaired Student t test. **P < .01; ***P < .001. Ctrl, control; ns, not significant.