External validity of a population-based study on osteoporosis and fracture

Abstract

Background and purpose — Little is known about the characteristics of non-participants in epidemiological studies. We evaluated external validity by comparing fracture and mortality rate in participants and non-participants in a longitudinal study on risk factors for fracture.

Methods — 1,604 randomly selected women, 75 years of age, were invited to attend a study on osteoporosis and fracture. 1,044 women attended the study (participants) and 560 women did not participate (non-participants). Fracture data for all were obtained prospectively from radiographic records. Mortality data were obtained through the population register. Mean follow-up was 13 (11–15) years. Cumulative survival was compared with the log-rank test. Fracture incidence rates per 1,000 person-years were compared with Mann-Whitney U-tests. In addition, fracture comparisons were made with the cumulative incidence function and Gray’s test.

Results — 454 participants (44%) died during the follow-up, as compared to 372 of the non-participants (66%) (p < 0.001). The fracture incidence rate for any type of fracture was 43 for participants and 47 for non-participants (p = 1.0). The fracture incidence rate for typical osteoporotic fracture was 36 for participants and 39 for non-participants (p = 0.6). The corresponding values for distal forearm fracture were 11 and 7 (p = 0.002), they were 8 and 9 for proximal humerus fracture (p = 0.9), 13 and 10 for vertebral fracture (p = 0.007), 15 and 18 for hip fracture (p = 0.8), and they were 6 and 5 for pelvic fracture (p = 0.3). The cumulative incidence function confirmed the results.

Interpretation — Our findings suggest that participants had a lower mortality rate than non-participants. Distal forearm and vertebral fractures were more frequent in participants. However, the external validity for fractures in general appeared to be satisfactory.

AW: acquisition of data; design, analysis, and interpretation of data; and drafting and revision of the manuscript. KÅ: acquisition of data and revision of manuscript. PG: conception and design; acquisition of data; and revision of manuscript. Statistical analysis was done in collaboration with Per Näsman, KTH Royal Institute of Technology, Stockholm, Sweden.

Funding was obtained from the regional agreement on medical training and clinical research (ALF) between Stockholm County Council and Karolinska Institutet, and from Karolinska Institutet Research Funds, the Swedish Research Council (K2009-53X-14691-07-3, K2010-77PK-21362-01-2), FAS (Grant 2007-2125), the Greta and Johan Kock Foundation, the A. Påhlsson Foundation, A. the Osterlund Foundation, the Knut and Alice Wallenberg Foundation, the H Järnhardt Foundation, the King Gustav V and Queen Victoria Foundation, the Malmö University Hospital Research Foundation, and the Research and Development Council of Region Skåne, Sweden.

No competing interests declared.