Step-by-step diagnostic workup depending on ICAHT severity. ∗In case of elevated ferritin and clinical suspicion of MAS, refer to supplemental Table 3 and supplemental Figure 1. AML, acute myeloid leukemia; CMV, cytomegalovirus; CRS/MAS, cytokine release syndrome with macrophage activation syndrome; EBV, Epstein-Barr virus; GPI, glycosylphosphatidylinositol; HHV6, human herpesvirus 6; HSV, herpes simplex virus; IgG, immunoglobulin G; JCV, JC virus; MDS, myelodysplastic syndrome; MPN, myeloproliferative neoplasm; NGS, next-generation sequencing; PCR, polymerase chain reaction; PNH, paroxysmal nocturnal hemoglobinuria.