Purpose: The purpose of this study was to use the Catquest-9SF to measure cataract surgery outcomes, and to use Rasch analysis to test the psychometric properties of this questionnaire, including its validity and responsiveness.

Methods: Patients were recruited as consecutive cataract surgery patients during 1 month at six surgical units in Sweden (via the National Cataract Register). The patients completed the questionnaire before surgery and 3 months after. The Catquest-9SF data were assessed for fit to the Rasch model using version 3.63.2 of the WINSTEPS software (Winsteps.com, Beaverton, OR, USA). Both preoperative and postoperative questionnaires were included in the analysis. The responsiveness to cataract surgery was calculated as the effect size.

Results: Completed questionnaires before and after surgery were received from 846 patients. The Rasch analysis showed that the category thresholds were ordered. All items fit a single overall construct (infit range 0.79–1.40; outfit range 0.74–1.40). The ability to discriminate different strata of person ability was good, with a real patient separation of 2.58 and patient separation reliability of 0.87. The questionnaire showed unidimensionality and was largely free from differential item functioning. The item difficulty was reasonably well targeted to both preoperative and postoperative patient ability. The Catquest-9SF Rasch score correlated significantly with visual acuity, and cataract surgery resulted in a significant improvement with an effect size of 1.8.

Conclusions: The Catquest-9SF shows excellent psychometric properties, as demonstrated by Rasch analysis. It is highly responsive to cataract surgery, and its brevity (nine items) makes it well suited for use in daily clinical practice.

Key Words: cataract surgery, questionnaire, Rasch analysis, response theory, validation, visual disability, visual function

Acta Ophthalmologica 2009
Received on 7 June 2009. Accepted on 11 October 2009.

Mats Lundstrom¹, Anders Behndig², Maria Kugelberg³, Per Montan³, Ulf Stenevi⁴ and Konrad Pesudovs^5
1EyeNet Sweden, Blekinge Hospital, Karlskrona, Sweden
²Department of Ophthalmology, Umeå University Hospital, Umeå, Sweden
³St. Erik Eye Hospital, Stockholm, Sweden
⁴Department of Ophthalmology, Sahlgrenska University Hospital, Mölndal, Sweden
⁵NH&MRC Centre for Clinical Eye Research-Flinders University and Flinders Medical Centre, Adelaide, South Australia, Australia.