Flow cytometric detection of intraplatelet munc13-4 protein expression effectively identifies FHL3 patients. (A) Flow cytometric analysis of intraplatelet munc13-4 expression in FHL3 patients and healthy controls (Ctrl). Blue histograms represent staining with the anti-munc13-4 antibody; red histograms represent staining with isotype control Abs. P4, P11, P13, and P15 received platelet transfusions (Tr) several days before blood samples were taken. (B) Munc13-4 expression levels in platelets from enrolled patients relative to that of controls. Each dot shows the relative mean fluorescence intensity (MFI) of munc13-4 protein expression in a patient sample compared with that in a control sample: (MFI of munc13-4 staining − MFI of isotype staining) in a patient/(MFI of munc13-4 staining − MFI of isotype control staining) in a control. Five patients with FHL2, 1 patient with X-linked lymphoproliferative syndrome 1 (XLP1), and 1 patient with XLP2 were included in the non-FHL3 (nFHL3) group. Data from 4 FHL3 and 21 nFHL3 patients who received a transfusion within several days of sampling are shown separately. (C) Receiver operating characteristics curve analysis of relative munc13-4 MFI to discriminate FHL3 patients from those with other types of HLH. Patients who received a platelet transfusion were excluded from the analysis. Error bars indicate the mean ± standard deviation (SD). *P < .05; **P < .01; and ***P < .001. AUC, area under the curve.