Prognostic value of LRPAP1 autoantibody. (Ai) After a median follow-up of 6.8 years, for LRPAP1 autoantibody–seropositive patients, the median FFS was not reached. Patients without LRPAP1 autoantibodies had a median FFS of 5 years (P = .0052). The MIPI score–adjusted hazard ratio for FFS was 0.48; 95% CI, 0.27-0.83; P = .0083. (Aii) Using a more restrictive definition of LRPAP1 autoantibody seropositivity by restriction to baseline samples (date of reception before or maximally 5 days after start of treatment), after a median follow-up of 6.5 years, for LRPAP1 autoantibody–seropositive patients, the median FFS was not reached. Patients without LRPAP1 autoantibodies had a median FFS of 4.8 years (P = .0417). MIPI score–adjusted hazard ratio: 0.51; 95% CI, 0.26-1.01; P = .053. (Bi) After a median follow-up of 6.8 years, for LRPAP1 autoantibody–seropositive patients, the median OS was not reached. Patients without LRPAP1 autoantibodies had a median OS of 10 years (P = .0142). MIPI score–adjusted hazard ratio: 0.47; 95% CI, 0.24-0.94; P = .032. (Bii) Sensitivity analysis using a more restrictive definition of LRPAP1 autoantibody seropositivity by restriction to baseline samples (date of reception before or maximally 5 days after start of treatment), after a median follow-up of 6.5 years, for LRPAP1 autoantibody–seropositive patients, the median OS was not reached. Patients without LRPAP1 autoantibodies had a median OS of 8 years (P = .0109). MIPI score–adjusted hazard ratio: 0.31; 95% CI, 0.11-0.85; P = .023.