Abstract:
Tophi are comprised of monosodium urate (MSU) crystals embedded in inflammatory and fibrous tissue. The formation of tophi is a result of prolonged hyperuricemia. Subcutaneous tophi are typically visible during the advanced stage of gout. Currently, tophus regression is measured by physical assessments such as with Vernier calipers and tape measurement. However, there is a need for a patient reported outcome (PRO) tool that measures the burden of tophi. The aim of this study was to develop a PRO tool to assess tophus burden in patients with tophaceous gout. Twenty-five people with tophaceous gout were interviewed to explore key concerns associated with tophus burden. The responses from these interviews facilitated the generation of the preliminary PRO tool. Cognitive testing of the preliminary tool was performed to ensure face and content validity. Five healthcare professionals and five patients with tophaceous gout were interviewed to examine the items. The Tophus Severity Questionnaire (TSQ) was developed as a result of this process. The TSQ was administered to 103 patients with tophaceous gout. Psychometric testing of the TSQ was performed to ensure that the PRO tool was linear, unidimensional and fitted the Rasch model. Statistical analyses were performed to examine construct validity and reproducibility. Psychometric testing of the TSQ derived the 20-Item Tophus Impact Questionnaire (TIQ-20) and the subsequent Short Version of the Tophus Impact Questionnaire (TIQ-11). The TIQ-20 and TIQ-11 were found to have acceptable psychometric properties. Specifically, these were the Rasch model requirements of unidimensionality, linearity, precision, differential item functioning analysis, items residual correlations and persons separation index. However, the TIQ-20 had greater face validity, higher test-retest reliability and greater concurrent validity. This work has developed and validated two tophus-specific PRO in a population of patients with gout. The TIQ-20 and the TIQ-11 demonstrated acceptable psychometric properties. Initial results showed promising construct validity, reliability and feasibility. Further research is required to determine the responsiveness to change in tophus impact over time and between different interventions for the TIQ-20 and TIQ-11.