Psychometric validation of the quality of life Inventory - Disability (QI-Disability) among patients with Lennox-Gastaut syndrome and Dravet syndrome.

BACKGOUND: To evaluate the psychometric properties of the Quality of Life Inventory -Disability (QI-Disability) for individuals with Dravet syndrome (DS) or Lennox-Gastaut syndrome (LGS), two rare developmental and epileptic encephalopathy conditions.

METHODS

Cross-sectional data for individuals were drawn from the Adelphi DS & LGS Disease Specific Programme including the caregiver-reported QI-Disability and EQ-5D-5L, and physician-reported (7-point Likert scale) rating or quality of life. Factor structure of the QI-Disability was assessed using confirmatory factor analysis with goodness-of-fit statistics and internal consistency was assessed using Cronbach's alpha. Convergent validity was assessed by evaluating relationships between QI-Disability, EQ-5D-5L scores, and physician QOL ratings.

RESULTS

There were 154 patients with DS and 196 patients with LGS. The mean (SD) total QI-Disability score was 57.3 (SD 11.9) for DS and 58.9 (SD 13.8) for LGS. Fit statistics for the 6-factor QI-Disability model were mostly satisfactory; the factor loading for one item was unsatisfactory. Good internal consistency (alpha > 0.7) was found for all QI-Disability domains and for the total score in both LGS and DS groups. Convergent validity was demonstrated with correlations being as expected between QI-Disability and EQ-5D-5L scores, for example, strong correlations between the QI-Disability Physical Health and EQ-5D pain/discomfort dimension scores. There was a mean increase of approximately 3 points in the QI-Disability total score per unit category change in the physician-rated QOL scale.

CONCLUSIONS

QI-Disability had mostly satisfactory evidence of validity and reliability for DS and LGS and appears suitable for use in clinical practice and clinical trials.