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

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.

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.

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.

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.