Screening for osteopenia and osteoporosis: selection by body composition

Abstract

There is a great need for simple means of identifying persons at low risk of developing osteoporosis, in order to exclude them from screening with bone mineral measurements, since this procedure is too expensive and time-consuming for general use in the unselected population. We have determined the relationships between body measure (weight, height, body mass index, lean tissue mass, fat mass, waist-to-hip ratio) and bone mineral density (BMD) in 175 women of ages 28-74 years in a cross-sectional study in a county in central Sweden. Dual-energy X-ray absorptiometry was performed at three sites: total body, L2-4 region of lumbar spine, and neck region of the proximal femur. Using multiple linear regression models, the relationship between the dependent variable, BMD, and each of the body measures was determined, with adjustment for confounding factors. Weight alone, in a multivariate model, explained 28%, 21% and 15% of the variance in BMD of total body, at the lumbar spine and at the femoral neck according to these models. The WHO definition of osteopenia was used to dichotomize BMD, which made it possible, in multivariate logistic regression models, to estimate the risk of osteopenia with different body measures categorized into tertiles. Weight of over 71 kg was associated with a very low risk of being osteopenic compared with women weighing less than 64 kg, with odds ratios (OR) of 0.01 (95% confidence interval (CI) 0.00-0.09), 0.06 (CI 0.02-0.22) and 0.13 (CI 0.04-0.42) for osteopenia of total body, lumbar spine and femoral neck, respectively. Furthermore a sensitivity/specificity analysis revealed that, in this population, a woman weighing over 70 kg is not likely to have osteoporosis. Test specifics of a weight under 70 kg for osteoporosis (BMD less than 2.5 SD compared with normal young women) of femoral neck among the postmenopausal women showed a sensitivity of 0.94, a specificity of 0.36, positive predictive value (PPV) of 0.21, and negative predictive value (NPV) of 0.97. Thus, exclusion of the 33% of women with the highest weight meant only that 3% of osteoporotic cases were missed. The corresponding figures for lumbar spine were sensitivity 0.89, specificity 0.38, PPV 0.33, and NPV 0.91. All women who were defined as being osteoporotic of total body weighed under 62 kg. When the intention was to identify those with osteopenia of total body among the postmenopausal women we attained a sensitivity of 0.92 and a NPV of 0.91 for a weight under 70 kg, whereas we found that weight could not be used as an exclusion criterion for osteopenia of femoral neck and lumbar spine. Our data thus indicate that weight could be used to exclude women from a screening program for postmenopausal osteoporosis.