BACKGROUND:Carcinoid syndrome is associated with a reduced quality of life that can be attributed to symptoms such as diarrhea and fatigue as well as social and financial issues. This study was conducted to psychometrically assess meaningful change in bowel movement frequency among carcinoid syndrome patients using data from the TELESTAR clinical study. METHODS:An anchor-based approach for deriving meaningful change thresholds consisted of mapping change from baseline bowel movement frequency to other patient-reported assessments of change. These included the European Organization for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire - Core Questionnaire (QLQ-C30) Diarrhea Symptom responders, the EORTC Gastrointestinal NET questionnaire (GI.NET21) GI Symptom responders, and reported adequate relief at Week 12 (≥ 10-point score decrease from Day 1 to Week 12). Parameters included within-group mean change from baseline to Week 12, t-tests of the change (Wilcoxon rank sum for adequate relief), and effect size. RESULTS:There were 135 carcinoid syndrome patients with a mean baseline frequency of 5.7 bowel movements a day. A distribution-based method yielded meaningful change estimates of 0.62 bowel movements a day for overall frequency and 0.83 bowel movements a day at Week 12. Anchor-based analysis indicated a large effect size among patients who reported adequate relief at Week 12 (- 1.58; n = 18; P = 0.014), the QLQ-C30 Diarrhea domain responders (- 1.24; n = 40; P < 0.001), and the GI.NET21 GI Symptoms Domain responders (- 1.49; n = 25; P = 0.005). Exit interview data for meaningful change yielded effect size estimates of - 1.57 for overall change during the Double-blind Treatment Period and - 1.97 for change between Baseline and Week 12. CONCLUSIONS:Meaningful change derivation is critical to interpret patient outcomes for evaluating treatment efficacy. In this study, carcinoid syndrome patients experienced clinically meaningful reductions in bowel movement frequency of ≥30% over 12 weeks with telotristat ethyl treatment. TRIAL REGISTRATION:NCT01677910 .