Development of a Keratoconus-Specific Questionnaire Using Rasch Analysis.

PURPOSE: To develop and validate a keratoconus-specific quality of life (QoL) questionnaire: the Keratoconus Outcomes Research Questionnaire (KORQ).

METHODS

The study was carried out in three phases. Phase I: content identifications: items were identified based on an extensive literature review, open-ended patient mail survey, and expert consultations. Each item was scored on a visual analog scale (VAS). Phase II: pilot testing using Rasch analysis. Phase III: testing psychometric properties of the final version of the KORQ.

RESULTS

Phase I identified 44 items across 3 different content areas: activity limitation (26), symptoms (20), and convenience (8). The 44-item KORQ was self-administered to 158 people with keratoconus. The 44-item KORQ was multidimensional. Unidimensionality was restored by separating items across three content areas (subscales) as identified in phase I. The activity limitation and symptoms subscales demonstrated adequate measurement precision, but convenience (precision, 1.01) did not. Hence, the convenience subscale was discarded. Rasch analysis revealed that the VAS was disordered. The ordering of the VAS was restored by collapsing categories into 4. An iterative Rasch analysis guided item-removal resulted into a 29-item KORQ (18-item activity limitation and 11-item symptoms). The VAS was replaced by a discrete 4-option labeled categorical rating scale, and it was self-administered by 169 people with keratoconus. Both the subscales demonstrated good psychometric properties. The KORQ scores strongly correlated with visual acuity and contrast sensitivity demonstrating its construct validity.

CONCLUSIONS

The 29-item KORQ was a psychometrically robust and valid instrument to assess the impact of keratoconus on activity limitation and symptoms.