Psychometric properties of the short form of the Stroke Impact Scale in German-speaking stroke survivors

Background The short form of the Stroke Impact Scale (SF-SIS) consists of eight questions and provides an overall index of health-related quality of life after stroke. The goal of the study was the evaluation of construct validity, reliability and responsiveness of the SF-SIS for the use in German-speaking stroke patients in rehabilitation. Methods The SF-SIS, the Stroke Impact Scale 2.0 (SIS 2.0), EQ-5D-5L, National Institutes of Health Stroke Scale (NIHSS) and de Morton Mobility Index were assessed in 150 inpatients after stroke, with a second measurement two weeks later for the analyses of responsiveness. In 55 participants, the test-retest-reliability was assessed one week after the first measurement. The study was designed following the recommendations of the COSMIN initiative. Results The correlations of the SF-SIS with the SIS 2.0 (ρ = 0.90), as well as the EQ-5D-5L (ρ = 0.79) were high, as expected. There was adequate discriminatory ability of the SF-SIS index between patients who were less and more severely affected by stroke, as assessed by the NIHSS. Exploratory factor analysis indicated a two-factor structure of the SF-SIS explaining 59.9% of the total variance, providing better model fit in the confirmatory factor analysis than the one-factorial structure. Analyses of test-retest-reliability showed an intraclass correlation coefficient of 0.88 (95% CI 0.75-0.94). Hypotheses concerning responsiveness were not confirmed due to lower correlations between the assessments change scores. Conclusion Results of this analysis of the SF-SIS's psychometric properties are matching with the validity analysis of the English original version, confirming the high correlations with the Stroke Impact Scale and the EQ-5D-5L. Examination of structural validity did not confirm the presumed unidimensionality of the scale and found evidence of an underlying two-factor solution with a physical and cognitive domain. Sufficient test-retest reliability and internal consistency were found. In addition, this study provides first results for the responsiveness of the German version. Trial registration The study was registered at the German Clinical Trials Register. Trial registration number DRKS00011933, date of registration: 07.04.2017.