AbstractBackgroundA realistic treatment goal for myasthenia gravis (MG) is achieving minimal manifestations or better status with prednisolone at ≤5 mg/day (MM‐or‐better‐5 mg), considering a patient's health‐related quality of life. Prognosis prediction during the early phases of immunotherapies might be critical for determining subsequent treatment strategies; however, the appropriate biomarkers remain unknown.AimThis study aimed to clarify whether the reduction rate of anti‐acetylcholine receptor antibody (RR‐AChR Ab) titer levels is a useful biomarker for predicting MM‐or‐better‐5 mg achievement.MethodsWe retrospectively investigated patients with MG and AChR Abs who received immunotherapy for the first time. The RR‐AChR Ab titer levels were calculated in the early (within 30 days), middle (31–60 days), and late (61–100 days) periods after starting immunotherapies. A receiver operating characteristic (ROC) curve was generated to determine an appropriate cutoff value for RR‐AChR Abs to achieve an MM‐or‐better‐5 mg.ResultsOf 53 patients, 24 (45%) achieved MM‐or‐better‐5 mg after 1 year. For the early period, the RR‐AChR Ab cutoff value to predict MM‐or‐better‐5 mg was 1.68%/day with an area under the curve (AUC) of 0.75 (sensitivity, 85%; specificity, 70%). However, the middle and late posttreatment AUC values did not predict MM‐or‐better‐5 mg achievement.ConclusionThe RR‐AChR Ab might be an appropriate prognostic biomarker during the early period of MM‐or‐better‐5 mg achievement. In the era of early fast‐acting treatment strategies, the RR‐AChR Ab trend after starting immunotherapies may guide the subsequent treatment choices.