Complex diseases, such as systemic lupus erythematosus (SLE), present dilemmas over choice of outcome measures. Using a battery of instruments to capture the impact of different impairments or activity limitations experienced does not provide assessment of the wider impact on quality of life (QoL). This paper describes the development and testing of a new instrument to measure QoL in systemic lupus erythematosus (L-QoL).
The development combines theoretical strengths of the needs-based QoL model with statistical and diagnostic powers of the Rasch model. Content was derived from in-depth interviews with relevant patients. Cognitive debriefing interviews assessed face and content validity. Rasch analysis was applied to data from an initial postal survey to remove misfitting items. A second postal survey assessed scaling properties, reliability, internal consistency and validity.
A 55-item questionnaire was derived from interview transcripts. Cognitive debriefing confirmed acceptability. Rasch analysis of postal survey data (n = 95) removed misfitting items. A second postal survey (n = 93), produced a 25-item version with good item fit and stability, excellent test-retest reliability (0.92), internal consistency (0.92) and strict unidimensionality.
It is concluded that the L-QoL should prove a valuable instrument for assessing patient-based outcome in clinical trials and practice.