Adaptation and Validation of the Systemic Sclerosis Quality of Life Questionnaire into Polish: Item Analysis Using Rasch Models

Background Systemic Sclerosis SSc is a multisystem autoimmune disease with serious effects on patients' Quality of Life (QoL). SScQoL questionnaire was developed in the in the UK to measure needs-based patient-reported quality of life in SSc.1

Objectives The aim of this study was to adapt and validate the SScQoL for use in Poland.

Methods This cross-sectional study involved the (i) adaptation phase and (ii) validation phase. In the adaptation phase, the SScQoL was translated into Polish using a standard process for adaptation of self-report quality of life measures.2 In the validation phase, patients with SSc were recruited from 8 rheumatology centres in Poland and they were asked to complete the Polish version of the SScQoL. The data was then analysed using Rasch models to determine fit to the model, local dependency, internal consistency, unidimensionality and psychometric equivalence with other European versions of the SScQoL.

Results The SScQoL translated well into Polish. The sample included 231 patients, 198 (86%) of whom were women. Their median age was 58 (IQR: 48 to 64) years, median disease duration, 6 (IQR: 3 to 12) years and the majority left school at age 19. The analysis of the 29 items revealed lack of fit to the Rasch model and suggested some multidimensionality (Table 1). Items were grouped into 5 subscales derived from components of the International Classification of Functioning, Disability and Health. Re-analysis of the 5-subscale SScQoL resulted into a valid, reliable and unidimensional scale (only 4.3% of independent t-tests were significant).3 The scale was found to have measurement equivalence with five other European countries as described elsewhere.