INTRODUCTION: The 15-STARS questionnaire (Screening Tool for AdheRence to medicineS) is a validated self-report screening tool that assesses modifiable determinants of medication non-adherence. It was applied in the 5C-study with chronic inflammatory bowel diseases (IBD) patients in Basel, Switzerland during the winters 2022/23 and 2023/24. Its usefulness, defined as data quality, actionability, and accuracy has not yet been fully explored. The aims of this study were to assess i) which determinants are captured by the 15-STARS questionnaire in IBD patients, ii) corresponding adherence-enhancing interventions, and iii) the positive predictive value (PPV) of the questionnaire. METHODS: We performed a subset-analysis of the 5C-study using a mixed-method analysis. Data were analyzed quantitatively, and participants with presumed non-adherence were interviewed between November 2023 and April 2024. Interviews were analyzed inductively using thematic analysis. Interventions were defined, missing data and PPV were calculated. RESULTS: Of 144 participants, 139 returned the 15-STARS questionnaire. Presumed non-adherence was identified in 71 participants (49.3 %) and 55 interviews (38.2 %) were performed and analyzed. Forgetfulness (31.0 %) was the most frequently reported determinant, followed by problems with medicine use (26.8 %), difficulties with medication recognition or preparation (14.1 %). Interviews revealed unsuspected aspects such as patients focusing on on-demand therapy. The PPV was 68.5 % (item-based) respective 67.3 % (participant-based), and adherence-enhancing interventions were developed for all 55 patients. CONCLUSION: Our results confirm the high data quality, the accuracy and the possibility to develop specific patient-centered interventions with the 15-STARS questionnaire, demonstrating its usefulness in clinical practice with IBD patients. The tool might represent a real practical aid to detect relevant adherence barriers to chronic medicines use in daily practice. Future research could investigate its implementation in community and clinic pharmacy sites and explore its applicability with broader populations.