Date Published: April 22, 2019
Publisher: Public Library of Science
Author(s): Petr Kutáč, Václav Bunc, Martin Sigmund, Cherilyn N. McLester.
Dual-energy X-ray absorptiometry (DXA) is rapidly becoming more accessible and popular as a technique to monitor body composition. The reliability of DXA has been examined extensively using a number of different methodological approaches. This study sets up to investigate the accuracy of measuring the parameters of body composition (BC) by means of the whole-body and the segmental DXA method analysis with the typical error of measurement (TEM) that allows for expressing the error in the units of measure. The research was implemented in a group of 63 participants, all of whom were university students. Thirty-eight males (22.6±2.9 years, average body mass 77.5±8.4 kg) and 25 females (21.4±2.0 years, average body mass 58.6±7.2 kg) were recruited. The measured parameters included body mass (BM), fat-free mass (FFM), body fat (BF), bone mineral content (BMC), bone mineral density (BMD). For the whole-body analysis, the determined TEM was: BM at the level of 0.12 kg in females and 0.29 kg in males; BF 0.25kg and 0.44% females, 0.52 kg and 0.66% males; FFM 0.24 kg females and 0.42 kg males; BMC 0.02 kg females and males; BMD 0.01g/cm2 females and males. The TEM values in the segmental analysis were: BF within the range of 0.04–0.28 kg and 0.68–1.20% in females, 0.10–0.36 kg and 0.72–1.94% in males; FFM 0.08–0.41 kg females and 0.17–0.86 males, BMC 0.00–0.02 kg females and 0.01–0.02 kg males in relation to the body segment (upper limb, trunk, lower limb). The BMD value was at the level of 0.01–0.02g/cm2. The study results showed high reliability in measuring body composition parameters using the DXA method. The whole-body analysis showed a higher accuracy of measurement than the segmental. Only the changes that are greater than the TEM, or the upper bound (95%) of the confidence interval of the measurement can be considered demonstrable when interpreting repeated measurements.
The analysis of body composition has become common in the assessment of an organism’s condition. The values of body composition parameters are used to assess the health of an individual, the quality of nutrition and overall fitness. These values are also used in sports to assess the effects of training and nutrition on the changes in the individual components of the athlete’s body weight, or the changes in the individual components during the competition season [1–6]. To analyze body composition, indirect methods are used because a direct method would be difficult to execute in a living person. Currently, direct measurement of body segment inertial parameters (BSIPs) on living humans is possible using medical imaging technologies such as gamma-ray scanning [7,8], computed tomography imaging (CT) [9,10] and magnetic resonance imaging (MRI) [11–13]. Although accurate, they are not widely used in biomechanics due to costs, labor demands during data processing, limited accessibility and/or exposure of subjects to high doses of radiation. The indirect methods include field techniques and referential methods. The results of the analysis depend on the method and equipment used as well as the current condition of the individual [14–17]; in addition, the results obtained by the same method but different devices differ from one another [18–21]. The prediction equations for determining the measured parameters are the basic problem, each manufacturer uses their own equations in the device software. The results are influenced by the measurement errors. With regard to their effect, errors can be divided into systematic and random . A researcher cannot control random errors and systematic errors distort the result in the same way provided that the same conditions of measurement are observed. In addition to the inter- and intra-examiner errors, there are also errors related to the methodology, device and measuring instruments. Thus, to correctly interpret the results of the measurement (especially in repeated measurements), the knowledge of the errors in the chosen method and device is necessary. In the biomedical sciences field, it is recommended to express the error using the typical error of measurement (TEM) ; unlike the commonly used reliability coefficient, TEM allows for expressing the existing error directly in the units used in the experiment. To calculate TEM, Hopkins  recommends performing three repeated (consecutive) measurements.
The results present the mean input values of the whole-body and segmental analyses (the segmental analysis did not include the head) and the resulting values of the TEM. The mean input values were acquired in three repeated measurements of the individual participants, as described in the measurement procedure. The reproducibility of the measurement results using DXA was verified using TEM. The values of the whole-body analysis are presented in Table 1, the segmental analysis in Table 2. The results of the DXA measurement outcome reproducibility are presented in Tables 3 and 4. The TEM and ICC values, which are presented in Tables 3 and 4, are calculated as the mean values of two consecutive comparative measurements (Trial 1–2, 2–3).
The potential applications of determined TEM values can be demonstrated using results of studies that assess changes in body composition of athletes in various sports throughout the competition season [46–48]. In these studies, researchers assessed the changes in body composition using the DXA method in handball players in preseason and postseason , rugby players in preseason, midseason and end-season  and softball, basketball and volleyball players, swimmers, and track and field athletes in off-season, preseason and postseason . In many cases, the differences found, even when statistically significant, remain in a range of the TEM (or the upper bound of the confidence interval) or are very close to those values. In such cases, the changes present in the results could be considered negligible.
The positioning of the study participants and analysis methodologies implemented in the experiment resulted in very high, nearly perfect reliability when examining hard and soft tissue masses across all segments of the upper and lower extremities. No observable difference in reliability was evident between the upper-body and lower-body segments; hard-tissue masses achieved greater reliability than soft-tissue masses throughout. The percentage ratio of the resulting TEM measured parameter to the total value of the measured parameter showed that the whole-body DXA analysis provides a more accurate value than segmental analysis for both soft and hard tissues. Therefore, this should be respected when using this method in practice. The TEM values of soft tissues show a lower error of measurement in women than in men in the whole-body and segmental analysis. The position of the subject has to be precisely fixated to ensure reproducibility of the DXA analysis results. The knowledge of TEM values is critical when interpreting outcomes of repeated measurements. Only the changes that are greater than TEM, or the upper bound (95%) of the confidence interval of the measurement, can be considered true changes. The reproducibility of the DXA items determination showed no statistical difference between genders for the two measurements representing hard tissue.