Abstract
BACKGROUND: We aimed to develop a patient-reported outcome measure, in accordance with the US Food and Drug Administration guidance, to capture the impact of systemic sclerosis-related digital ulcers (SSc-DUs) on hand function. Psychometric analyses were conducted to evaluate and document the measurement properties of the resulting instrument-the Hand Disability in Systemic Sclerosis-Digital Ulcers (HDISS-DU®). METHODS: The HDISS-DU was developed through a series of confirmatory, qualitative concept-elicitation interviews (N = 36) to provide supportive evidence that the instrument captures all relevant issues and functional limitations relating to SSc-DUs in this patient population. Psychometric analyses used blinded data from two randomised, controlled, phase 3 trials in patients with SSc-DUs (N = 517). The analyses included assessment of reliability, construct validity, responsiveness and thresholds for meaningful change. RESULTS: Qualitative interviews confirmed that the HDISS-DU had good content coverage and patients understood the HDISS-DU instructions, items and response scale. The HDISS-DU demonstrated excellent internal consistency and test-retest reliability, with satisfactory construct validity. Overall, the HDISS-DU was highly responsive to change in digital ulcer severity: the no-change group (for other criterion measures) had mean differences and effect sizes close to 0, while mean differences were mostly negative (indicating improvement) for the improvement groups (for other criterion measures) and vice versa. The preliminary threshold for meaningful change was a 0.50 difference in HDISS-DU score. CONCLUSIONS: Using data from two large studies of SSc-DU patients, these psychometric analyses support the reliability, validity, discriminating ability and responsiveness to change of the HDISS-DU for evaluating treatment outcomes in future clinical studies and clinical practice.