March 1998, Volume 52, Issue 3, Pages 180 – 185

Original Communication

Comparison of in-vivo body composition using two Lunar dual-energy X-ray absorptiometers B Oldroyd1,3, JG Truscott1, G Woodrow2, R Milner1, SP Stewart1, AH Smith1, CF Westmacott1 & MA Smith1 1Centre for Bone and Body Composition Research, University of Leeds     2Renal Unit, The General Infirmary at Leeds, Leeds, UK     3Correspondence: Mr B Oldroyd, Higher Scientific Officer, Centre for Bone and Body Composition Research, Wellcome Wing, The General Infirmary, Leeds LS1 3EX, UK.

Keywords

Dual energy X-ray absorptiometry;   quality assurance;   body composition;   fat mass;   lean mass;   bone mineral

Abstract

Objective: To compare in-vivo composition analysis between two dual energy X-ray absorptiometers, a DPX and a DPX/L, from the same manufacturer (LUNAR), pre(Study A) and post(Study B) hardware changes on both absorptiometers. Design: Comparison of (1) quality assurance (QA) data: air-counts low (38 keV), air-counts high (70 keV), air-counts ratio, percent spillover, R-delrin; and (2) total body compartments: total body tissue (TBTISS), total body fat (TBF), percent total body fat (%TBF), total body lean (TBLEAN), total body bone mineral content (TBBMC) and total body bone mineral density (TBBMD), between the two absorptiometers. Setting: Centre for Bone and Body Composition Research, University of Leeds. Subjects: Study A, 14 normal subjects and Study B, a different cohort of 19 normal subjects, were scanned on both machines on the same day. Results: In Study A, large significant differences were observed in the QA parameters between the two machines. The DPX, air-counts low and air-counts high, being 25% and 22% lower than the DPX/L. The Bland-Altman method of analysis indicated that the DPX was significantly higher for TBTISS (0.3 kg), %TBF (2%) and TBF (1.4 kg) and correspondingly lower for TBLEAN (-1.0 kg). No significant difference was observed in TBBMC. After the hardware changes (Study B) a marked reduction in the differences in QA air-counts was observed. The DPX air-counts low was now 1% higher and air-counts high 8% lower than the DPX/L. The DPX had now only small significant negative differences for %TBF (-0.6%) and TBF (-0.4 kg) and a small significant positive difference for TBLEAN (0.4 kg), compared to the DPX/L. TBBMC difference although slightly increased, was still non-significant. Conclusions: The closer agreement observed in the QA parameters after the hardware changes was associated with a reduction in the mean differences, 95%CI of the mean differences and limits of agreement of the comparison of body composition analysis from the Lunar machines using the Bland-Altman method. The study indicates that the QA limits set for bone mineral analysis may require more stringent limits for body composition. Sponsorship: The study was supported by funding from the Medical Research Council, the University of Leeds and the United Leeds Teaching Hospitals Special Trustees.

Received 2 June 1997; Accepted 25 October 1997

^© Macmillan Publishers Ltd 1997