Validity and test retest reliability of the vascular quality of life Questionnaire-6: a short form of a disease-specific health-related quality of life instrument for patients with peripheral arterial disease

BACKGROUND: Many existing patient-reported outcome measures are extensive regarding both patient burden and administration, and in terms of analysing and reporting results. The VascuQoL-6 (VQ6) – a short version of the original Vascular Quality of Life Questionnaire (VascuQoL), a disease-specific instrument for peripheral arterial disease – was recently developed. However, the VQ6 has not yet been empirical tested with regard to content validity, construct validity and test retest reliability. Our aim was, therefore, to explore both the validity and the reliability of the VQ-6 in a target population with established peripheral arterial disease. METHODS: Two hundred patients treated at two vascular centres were consecutively recruited for the survey. Administered questionnaires included VQ6 and the Short Form Health Survey-36 (SF-36). Out of the 200 patients, 150 also received a second VQ6 questionnaire for a test-retest assessment. Further, a purposive sample of 22 patients consented to participate in cognitive interviews. All included patients suffer from peripheral arterial disease. The questionnaire data was tested by both Rasch analysis and traditional psychometric methods, while the cognitive interviews were analysed descriptively. RESULTS: The validity and reliability of the VQ6, as tested in a target population without the surrounding 19 items from the original VascuQoL, was high, in general, and a good fit to the Rasch model was observed. Further, an excellent internal consistency and significant correlations between comparable dimensions in SF-36 were demonstrated. In the test-retest analysis, the percentage agreement was somewhat poor (<70%) in the six items. However, no systematic disagreements between the two assessments were seen in any of the six items, and the test-retest assessment for the VQ6 sum score showed an acceptable intraclass correlation coefficient (0.86). Finally, all items in the VQ6 were considered as both understandable and relevant by the interviewed patients. CONCLUSIONS: The VQ6 has acceptable to good psychometric properties with regard to data quality, scale assumptions, targeting, validity and reliability. Further, VQ6 seems to be easy to use and comprehend within the target population of patients with PAD.