Validation, test-retest reliability and norm scores for the Dutch Catquest-9SF

PURPOSE: The Catquest-9SF questionnaire is a unidimensional, reliable, valid and short patient-reported outcome measure for quantifying benefits in visual functioning from cataract surgery. Our aim was to develop a formal Dutch translation, calculate norm scores, assess its validity and test-retest reliability and provide an easy way for use in clinical practice.

METHODS: Translation of the questionnaire was performed according to guidelines of the International Society for Pharmacoeconomics and Outcomes Research. Catquest-9SF was obtained in 657 patients pre- and postcataract surgery. We applied Rasch and classical analyses to determine the questionnaire performance with characteristics such as unidimensionality, reliability, separation and differential item functioning. Test-retest reliability was assessed in another group of 145 patients. A cut-off value to discriminate between people with and without cataract, norm scores and a reliable change index (RCI) were calculated using data from a sample of 916 'healthy' persons from the normal population.

RESULTS: The Dutch Catquest-9SF was unidimensional, and both person and item reliability were high; 0.87 and 0.99, respectively. Cronbach's alpha was 0.94, test-retest reliability was 0.85 and the intraclass correlation coefficient was 0.93. Catquest-9SF showed to be responsive to the effect of cataract surgery (effect size = 1.27; p < 0.001). The cut-off value was -1.90, and RCI was 2.27. A quick-access table with norm scores and percentiles was established to facilitate clinical interpretation.

CONCLUSION: This investigation provides validity and reliability of the Dutch Catquest-9SF as well as norm scores and a new tool to facilitate the clinical interpretation of patient scores. This makes Catquest-9SF suitable for routine use in clinical practice.