B2M and diffuse large B-cell lymphoma: We aimed to (1) investigate the association of β2M with other clinically relevant prognostic factors in DLBCL registered in our national database, (2) compare four β2M‐based models with four International Prognostic Indices (IPI, age‐adjusted IPI [aaIPI], revised IPI [R‐IPI], NCCN‐IPI), and (3) investigate whether the addition of β2M to NCCN‐IPI would improve the discriminatory performance of NCCN‐IPI by developing a new model based on NCCN‐IPI variables and β2M.