ABSTRACT: INTRODUCTION:Online communities contain a wealth of information containing unsolicited patient experiences that may go beyond what is captured by guided surveys or patient-reported outcome (PRO) instruments used in clinical settings. This study described patient experiences reported online to better understand the day-to-day disease burden of ankylosing spondylitis (AS). METHODS:Unguided, English-language patient narratives reported between January 2010 and May 2016 were collected from 52 online sources (e.g., general/health social networking sites, patient-physician Q&A sites, AS forums). Using natural language processing combined with manual curation, patient-reported experiences within narratives were evaluated and categorized into social, physical, emotional, cognitive, and role activity (SPEC-R) concepts to assess functional impairment. The same SPEC-R categorization was applied to 5 AS-specific PRO instruments to evaluate their coverage of concepts extracted from patient narratives. RESULTS:A total of 34,780 narratives from 3449 patients with AS were included. Physical aspects of AS (e.g., pain and mobility) were most commonly reported by patients (86.7%), followed by emotional (32.5%), cognitive (23.6%), role activity (8.7%) and social (5.1%). Some frequently discussed subconcepts were effectively captured by ≥ 2 PRO instruments, such as pain (65.3%), asthenia (19.9%), musculoskeletal impairment (19.9%), depression (9.9%), and anger/frustration (5.4%); others [e.g., anxiety (19.1%), mental impairment (3.2%), impulsivity (2.9%)] were not addressed by any of the PRO instruments. CONCLUSION:These findings highlight the importance of analyzing patient experiences beyond clinical trial settings and physician reports; continuous assessment of existing PRO instruments in collaboration with patients may increase their utility in real-world settings. FUNDING:Novartis Pharmaceuticals Corporation.