title: Time related bias in longitudinal studies using dual photon absorptiometry.
abstract: Examining the bone mineral density (BMD's) slope of patients regularly followed in our department, we observed recently that the group of patients who had their last BMD during the last 6 months of 1989, had a different slope than patients who had their last BMD during the following 6 months. In order to investigate if a small time-related bias of measurement, unsuspected by the former quality control investigations, could exist, we performed the following analyses. A regression equation between BMD and time was calculated and a slope was obtained for 95 women who had been followed for at least 3 yr and had had at least 3 BMD measurements during that time. The women were divided in 3 groups according to when the last BMD measurement had been performed (July-December 1989, January-June 1990 or July-December 1990). The slopes of the 3 groups of patients were compared. For each value of BMD of every patient, a predicted BMD (BMDp) was calculated using the regression equation and the relative difference (RD) between BMDp and BMD was calculated and analysed in relation to time. There was a significant difference (p < 0.05) between the slopes of patients in relation to the time when the last BMD had been measured. Significant fluctuations (p < 0.001) in RD were observed in relation to time. These RD variations suggested the existence of a time-related error. The presence of this error is also substantiated by the fact that a parallelism existed between the curve of the RD variations and the curve of the mean values of BMD of all patients referred to our department, calculated per period of 4 months. Although the fluctuation of the latter curve was not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Interdisciplinary Group on Osteoporosis, Free Universities of Brussels (VUB-ULB), Hôpital Saint-Pierre, Belgium.
authors.forename: S
authors.initials: S
authors.lastname: Rozenberg
authors.forename: J
authors.initials: J
authors.lastname: Vandromme
authors.forename: A
authors.initials: A
authors.lastname: Peretz
authors.forename: J P
authors.initials: JP
authors.lastname: Praet
authors.forename: C
authors.initials: C
authors.lastname: Robyn
authors.forename: H
authors.initials: H
authors.lastname: Ham
mesh_terms: D015502:Absorptiometry, Photon; D000368:Aged; D000369:Aged, 80 and over; D015519:Bone Density; D005260:Female; D006801:Humans; D010024:Osteoporosis / Q000000981:diagnostic imaging; D013997:Time Factors