Original Article
In vivo Precision of the GE Lunar iDXA Densitometer for the Measurement of Total-Body, Lumbar Spine, and Femoral Bone Mineral Density in Adults

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Abstract

Knowledge of precision is integral to the monitoring of bone mineral density (BMD) changes using dual-energy X-ray absorptiometry (DXA). We evaluated the precision for bone measurements acquired using a GE Lunar iDXA (GE Healthcare, Waukesha, WI) in self-selected men and women, with mean age of 34.8 yr (standard deviation [SD]: 8.4; range: 20.1–50.5), heterogeneous in terms of body mass index (mean: 25.8 kg/m2; SD: 5.1; range: 16.7–42.7 kg/m2). Two consecutive iDXA scans (with repositioning) of the total body, lumbar spine, and femur were conducted within 1 h, for each subject. The coefficient of variation (CV), the root-mean-square (RMS) averages of SDs of repeated measurements, and the corresponding 95% least significant change were calculated. Linear regression analyses were also undertaken. We found a high level of precision for BMD measurements, particularly for scans of the total body, lumbar spine, and total hip (RMS: 0.007, 0.004, and 0.007 g/cm2; CV: 0.63%, 0.41%, and 0.53%, respectively). Precision error for the femoral neck was higher but still represented good reproducibility (RMS: 0.014 g/cm2; CV: 1.36%). There were associations between body size and total-body BMD and total-hip BMD SD precisions (r = 0.534–0.806, p < 0.05) in male subjects. Regression parameters showed good association between consecutive measurements for all body sites (r2 = 0.98–0.99). The Lunar iDXA provided excellent precision for BMD measurements of the total body, lumbar spine, femoral neck, and total hip.

Introduction

Dual-energy X-ray absorptiometry (DXA) is globally accepted as the recommended method for the diagnosis of osteoporosis and the evaluation of the effects of pharmaceutical interventions on bone density and osteoporosis prognosis. As with all medical devices, inherent variability exists with DXA scanning, and precision varies according to the skeletal region of interest. Knowledge of this precision error is essential to the clinical interpretation of patient results, because it is important to distinguish between true change in bone mineral density (BMD) and random variability in the measurement procedure 1, 2. This distinction is also important for sample size calculations in preparation for research studies and clinical trials.

The International Society for Clinical Densitometry (ISCD) recommends that in vivo precision error for each DXA center is to be determined to distinguish actual biological changes in BMD from random densitometer fluctuations 1, 2. The least significant change (LSC) enables the identification of statistically significant changes in BMD, by considering the precision error (coefficient of variation [CV]) of the measurement. Aside from manufacturer's (GE Healthcare, Waukesha, WI) values, precision errors for DXA of 1.0% for spine (L1–L4), 0.9–1.2% for the total femur, and 1.5–1.6% for the femoral neck have been reported for the Lunar Expert (3), Lunar Prodigy (4), and Hologic (5) machines. Differences in precision between DXA devices by model or manufacturer have been reported 3, 4. The iDXA (GE Healthcare, UK) is the latest model of fan-beam DXA from Lunar. The Lunar iDXA differs from previous models by improved point typing, producing images with higher resolution and accommodating larger patients (up to 182 kg). The aims of this study were to determine short-term, in vivo precision and LSC values for total-body, spine, femoral neck, and total-femur BMD, and total-body bone mineral content (BMC) in a cohort of men and women.

Section snippets

Subjects

In this study, 52 subjects (male n = 18; female n = 34) were measured twice to improve precision reliability. This results in 52 degrees of freedom (df) and a 95% confidence interval (CI) for the lower and upper error limits of the precision estimate of approx 0.84–1.24. The mean age of the subject group was 34.8 yr (standard deviation [SD]: 8.4; range: 20.1–50.5 yr). Subjects were recruited through a university-wide staff e-mail. All subjects provided signed informed consent and participated in the

Results

Descriptive results for the subject sample are shown in Table 1. The sample consisted of 34 female and 18 male subjects, and the subjects varied by ethnic group (Asian n = 10; Caucasian n = 42). Six subjects were measured using the thick-scan mode. According to standard body mass index (BMI) classification, 2 subjects were underweight (<18.5 g/cm2), 25 were of normal weight (18.5–24.9 g/cm2), 13 were overweight (25–29.9 g/cm2), 12 were obese (>30 g/cm2). The mean (SD) values obtained for each

Discussion

As the gold standard measurement for osteoporosis diagnosis, DXA has been shown to have good reproducibility 3, 7, 9. The GE Lunar iDXA is 1 of the most recent models of DXA and has improved resolution compared with previous models. Our study investigated the short-term precision of bone density measurements in a varied cohort of 52 men and women. We found excellent BMD measurement precision using the iDXA at the total body, lumbar spine, femoral neck, and total hip, and correlations were close

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This study was conducted without external funding/support.

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