Monitoring time interval for pQCT-derived bone outcomes in postmenopausal women
Evidence of measurement precision, annual changes and monitoring time interval is essential when designing and interpreting longitudinal studies. Despite the precise measures, small annual changes in bone properties led to monitoring time intervals (MTIs) of 2–6 years in peripheral quantitative computed tomography (pQCT)-derived radial and tibial bone area, density, and estimated strength in postmenopausal women. The purpose of the study was to determine the precision error, annual change, and MTI in bone density, area, and strength parameters in postmenopausal women. Postmenopausal women (n = 114) from the Saskatoon cohort of the Canadian Multicentre Osteoporosis Study had annual pQCT scans of the distal and shaft sites of the radius and tibia for 2 years. Median annualized rates of percent change and the MTI were calculated for bone density, area, and strength parameters. Root mean squared coefficients of variation (CV%) were calculated from duplicate scans in a random subgroup of 35 postmenopausal women. CV% ranged from 1.4 to 6.1 % at the radius and 0.7 to 2.1 % at the tibia. MTIs for the distal radius were 3 years for total bone density (ToD) and 4 years for total bone cross sectional area (ToA), trabecular area, and bone strength index. At the diaphyseal radius, MTI was 3 years for ToA, 5 years for cortical density, and 6 years for polar stress strain index (SSIp). Similarly, MTI for total and trabecular density was 3 years at the distal tibia. At the diaphyseal tibia, MTI for ToA was 3 years and SSIp 4 years. MTI for longitudinal studies in older postmenopausal women should be at least 2–6 years at the radius and tibia, with specific monitoring of the total and trabecular area, total density, and bone strength at the radius and total and trabecular density, total area, and bone strength at the tibia.