The natural progression of health-related quality of life: Results of a five-year prospective study of SF-36 scores in a normative population

Background: Limited information exists regarding the natural progression of health-related quality of life (HRQOL) in the general population, as most research has been cross-sectional or has followed populations with specific medical conditions. Such norms are important to establish, because the effect of any intervention may be confounded by changes due to the natural progression of HRQOL over time. Methods: Participants were randomly selected from 9 Canadian cities and surrounding rural areas. Changes in the eight domains and 2 summary component scores of the Medical Outcomes Study 36-item short form (SF-36) were examined over a 5 year period (1996/1997-2001/2002). Mean changes were calculated for men and women within 10 year age categories. Multiple imputation was used to adjust for potential selection bias due to missing data. Results: The baseline sample included 6539 women and 2884 men. Loss to follow-up was 17% for women and 23% for men. Mean changes tended to be small, but there was an overall trend towards decreasing HRQOL over time. Changes were more pronounced in the older age groups and in the physically oriented domains. Younger age groups tended towards small mean improvements, particularly in the mentally oriented domains. Large standard errors suggest that on an individual level, large improvements in some participants are balanced by large declines in others. Conclusion: In general, the HRQOL of Canadians appears relatively stable over a 5 year period. However, care should be taken when assessing HRQOL longitudinally in certain age or gender groups, as changes associated with an intervention can potentially be confounded by the natural progression of HRQOL.