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Introduction Digital ulcers (DU) remain a major cause of disease-related morbidity in systemic sclerosis (SSc). Previous clinical trials of SSc-DU have primarily focussed on clinician assessment of DU occurrence and healing. Previous trials have placed comparatively little emphasis on how patients ‘feel’ and ‘function’ (central to FDA drug approval), rendering them less useful. Furthermore, different aetiopathogenic drivers of SSc-DU have been proposed; however, little is known about how these modify the lived patient experience or natural history of SSc-DU.We have devised a novel patient reported outcome (PRO) measure: the Systemic Sclerosis Impact of Digital Ulcer (‘SSiDU’) questionnaire to capture the multifaceted severity and impact of SSc-DU. Our preliminary item bank comprises of 29 items, each of which is grounded in the five major themes that encapsulate the patient experience of SSc-DU identified in our earlier literature review and qualitative research.Our aims are to:-Elaborate and validate the novel SSc-DU PRO ‘SSiDU’ instrument.Understand the aetiopathogenic drivers of different types of SSc-DU, their relationship to the patient experience of SSc-DU, and benchmark DU healing (including impact of complications and treatment) in the modern era.Material and Methods The ‘100DU’ International multicentre longitudinal Study will recruit 100 SSc patients with active DU (including ischaemic, extensor and calcinosis-related ulceration) from a diverse geographic, cultural and ethnic sample of English-speaking patients.Results Patients will be enrolled from fourteen centres located within four continents (UK, USA, Canada and Australia). The ‘100DU’ Study will facilitate the comprehensive capture of patient-reported, clinician-assessed and imaging (photography and microvascular imaging) data. The below Table shows a summary of the study (visits and assessments) schedule. The pragmatic study design will include clinician assessments at 4-8 weeks and 18-22 weeks. The study will benefit from self-administered PRO questionnaire completion at home and additional face-to-face study visits (weeks 4 and 14) to mirror typical DU routine care. Prospective anchor/retrospective anchor questions will be used to establish Patient Acceptable Symptom States/Minimum Clinically Important Differences for the SSiDU questionnaire. Psychometric testing of the SSiDU will adhere to Outcomes in Rheumatology Clinical Trial (OMERACT) principles. Baseline nailfold capillaroscopy and thermographic assessments will be used to understand DU aetiopathogenic driversConclusions The ‘100DU’ Study primarily seeks to elaborate and validate our novel ‘SSiDU’ PRO instrument. We shall also capitalise on opportunities to learn novel insights into DU aetiopathogenesis and benchmark healing for future clinical trial design.Funding SCTC-SRF Betty Z. Benedict Award.Abstract P.188 Table 1