CAR T expansion has been linked to anti-tumor response in relapsed/refractory large B-cell lymphoma both in clinical trials and smaller real-world studies. Here, we present the largest multicenter real-world analysis to date, evaluating 262 patients treated with tisagenlecleucel or axicabtagene ciloleucel in second or subsequent relapse. Our findings underscore the complementary roles of multiparameter flow cytometry (MFC) and droplet digital PCR (ddPCR) in monitoring CAR T cells. While ddPCR accurately quantifies transgene copies, MFC provides critical phenotypic details, revealing CAR T-cell subpopulations that are associated with its efficacy. Consistent with prior studies, we confirm the association of CAR T expansion with response rates, progression-free survival (PFS), and toxicities. However, we reveal that expansion alone does not ensure efficacy. Elevated markers of systemic inflammation, such as ferritin and C-reactive protein (CRP), are linked to poorer outcomes despite robust expansion. These markers correlate with reduced cytotoxic CD8+ T cells with central memory features among in vivo expanded CAR T-cell populations, with similar associations observed in manufactured and leukapheresis products. Importantly, patients with high baseline inflammation who achieved significant expansion demonstrated PFS outcomes comparable to those with limited expansion, highlighting the negative impact of inflammation on CAR T-cell efficacy. Interestingly, ferritin and CRP levels were similar among responding patients, regardless of differences in CAR T expansion. Collectively, our findings indicate that systemic inflammation is associated with the phenotypic quality of T and CAR T cells. While functional validation is warranted, these results underscore the need to address inflammatory pathways to improve treatment outcomes.