CAR19 expansion is associated with CAR19-mediated toxicity. (A) CAR19 expansion defined by AUC is significantly associated with the development of severe ICANS. (B) Lower CAR19 CD4:CD8 ratio is also associated with the development of severe ICANS. (C) Increased CAR19 expansion is associated with more severe CRS. (D) Increased CAR19 expansion is associated with the need for G-CSF after D14. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001. All significance values represent Wilcoxon testing.