The effect of weight and weight change on the long-term precision of spine and hip DXA measurements.

SUMMARY: We examined the effect of weight and weight change on the long-term precision of spine and hip bone mineral density (BMD) in a group of 64 postmenopausal women studied over a 10-year period. Long-term precision errors were 50% larger than short-term errors. Over the range 50-90-kg weight was associated with a statistically significantly larger precision error when precision was expressed in BMD units, but not when expressed as the coefficient of variation (CV). Weight changes up to 5 kg had little effect on precision. INTRODUCTION: Reliable knowledge of the precision of bone mineral density (BMD) measurements is important for the interpretation of follow-up dual-energy X-ray absorptiometry (DXA) scans. In this study, we examined the effect of body weight and change in weight on the long-term precision of spine and hip BMD. METHODS: The study population was a group of 64 postmenopausal women enrolled in a 16-year trial of tibolone. We analyzed the spine, femoral neck, and total hip BMD data acquired over a 10-year period on a Hologic QDR4500A densitometer using linear regression to examine the trend of BMD with time for each subject. Precision was expressed in BMD units (g cm(-2)) (standard error of the estimate, SEE) and also as the coefficient of variation (CV). RESULTS: The long-term precision errors were in BMD (CV) units: 0.018 g cm(-2) (1.9%) for spine, 0.017 g cm(-2) (2.3%) for femoral neck, and 0.016 g cm(-2) (1.7%) for total hip BMD. An inverse relationship between CV and BMD was found for the spine (P = 0.003) and total hip (P = 0.043) sites, but none between SEE and BMD. For spine BMD, there were statistically significant correlations between SEE and weight (P = 0.025) and body thickness (P = 0.027). For femoral neck BMD, there were correlations between SEE and weight (P = 0.030), body mass index (BMI) (P = 0.023) and thickness (P = 0.021), but no correlations for total hip BMD or when precision was expressed as the CV. When study subjects were grouped in quartiles according to weight, the spine BMD SEE increased from 0.014 g cm(-2) for women in the lowest quartile (46-62 kg) to 0.018 g cm(-2) for women in the highest quartile (80-105 kg) (P = 0.008). There was a trend for SEE to be greater in individuals with larger weight changes, although these tended to be the heavier subjects. CONCLUSIONS: From the study, we were able to come up with the following conclusions: (1) long-term precision errors were 50% larger than short-term errors, (2) over the range 50 to 90 kg (BMI: 20-35 kg m(-2)), body weight had a small but statistically significant effect on precision expressed in BMD units, but not when expressed as the CV, and (3) weight changes up to 5 kg had little effect on precision. More studies of individuals >100 kg are required to fully investigate the dependence of DXA scan precision on weight.