Stability of normative data for the SF-36 - Results of a three-year prospective study in middle-aged Canadians

Background: The SF-36 is widely used to assess health-related quality of life (HRQOL), but with few longitudinal studies in healthy populations, it is difficult to quantify its natural history. This is important because any measure of change following an intervention may be confounded by natural changes in HRQOL. This paper assesses mean changes in SF-36 scores over a 3-year period in men and women between the ages of 40 and 59 years at baseline. Methods: Subjects were randomly selected from nine Canadian cities. Mean SF-36 changes over a 3-year period (1996/1997-1999/2000) were calculated for each gender within 5-year age categories. Multiple imputation was used to correct for potential bias due to missing data. Results: The baseline cohort included 1,974 women and 975 men between 40 and 59 years. Mean changes in HRQOL tended to be small. Women demonstrated small average declines in 22 of the 32 age and domain groupings (4 age groups, 8 SF-36 domains) while men showed declines in 14/32. Most participants stayed within 10 points of their original baseline score. Interpretation: Mean SF-36 scores change only slightly over three years in middle-aged Canadians, although there is much individual variation. It may be necessary to adjust for the natural evolution of SF-36 scores when interpreting results from longitudinal studies.