Changes in chronic pain severity over time: the Chronic Pain Grade as a valid measure.

Our understanding of the natural history of chronic pain in the community is limited. This is partly due to the lack of a validated measure of chronic pain severity known to be responsive to change over time. The Chronic Pain Grade questionnaire has been shown to be valid and reliable for use in a general population as a self-completion questionnaire. However, its reliability and validity for use in longitudinal studies and its responsiveness to change over time has not yet been assessed. We undertook a postal survey designed to test the responsiveness and the validity of the Chronic Pain Grade questionnaire over time. A random sample of 560 chronic pain patients, aged 25 years and over was drawn from an existing cohort and stratified for age, gender and chronic pain severity. Subjects were re-surveyed by a postal self-completion questionnaire consisting of the Chronic Pain Grade and the SF-36 general health questionnaire, which is known to be responsive to change in health over time. To test whether changes in CPG scores correlated with changes in SF-36 scores, Spearman's rank correlation coefficients were calculated. A response rate of 86% was achieved for the follow-up study. The majority of SF-36 scores changed in the hypothesized directions. Changes in CPG scores were significantly correlated with changes in most of the SF-36 domains. We concluded that the CPG is a useful and valid objective instrument for measuring change in severity of chronic pain over time and could be used in longitudinal studies of chronic pain severity.