Sarcopenia is characterized by a decrease in muscle mass and physical function with age and increases the risk of frailty, disability, falls, fractures, and death. As the average lifespan of Koreans continues to increase and the aging population accelerates, it is estimated that approximately 700,000 to 1 million elderly individuals over 65 years of age are affected by sarcopenia.[1-4] This condition is emerging as a significant health issue that escalates medical and long-term care costs, especially in a super-aged society.[5-8]

Since the first diagnostic criteria for sarcopenia were announced in 2010, numerous basic and clinical research results have been reported globally.[9-12] Recent updates include the 2019 European Sarcopenia Diagnosis and Management Guidelines and the 2020 Asian Sarcopenia Diagnosis and Management Guidelines.[13-15] These guidelines provide comprehensive recommendations for the diagnosis and management of sarcopenia, highlighting the importance of early detection and intervention.

Sarcopenia has been assigned a disease code in the International Classification of Diseases, Eleven Revision, and since 2021, the diagnosis of sarcopenia (M62.5) has been included in the 8th Korean Standard Disease Cause Classification.[16] With the international consensus on the diagnostic algorithms and criteria, and recognition as an official disease, there has been a global focus on the importance and value of sarcopenia.[9,17-19] This recognition underscores the need for standardized diagnostic approaches and effective treatment strategies to manage this growing health issue.

Dual energy X-ray absorptiometry (DXA or DEXA) is considered the gold standard for diagnosing sarcopenia; however, there is a need for comparative studies using bioelectrical impedance analysis (BIA), particularly in the Korean population.[20-23] DXA provides precise measurements of bone density and body composition but is limited by its high cost and accessibility issues. In contrast, BIA is a more affordable and accessible option, making it a valuable tool for widespread screening in clinical and community settings.

This study aimed to evaluate the correlation between the concordance of limb muscle mass between Hologic and GE Lunar, whole-body bone density-measuring instruments, and InBody 970, a bioelectrical impedance-measuring instrument. By assessing these correlations, the study seeks to determine if BIA can serve as a reliable alternative to DXA. Furthermore, it is necessary to understand the validity of the bioelectrical impedance measurement method for the diagnosis of sarcopenia. Such validation is crucial for adopting BIA in clinical settings, particularly for widespread and cost-effective screening of sarcopenia.

A total of 119 participants were recruited, considering their age, sex, and BMI ratio, as outlined in Table 1. The analysis was conducted in two main contexts.

When comparing the Inbody and DXA measurements, a meticulous assessment of the muscle mass was performed. This involved using BIA with the InBody 970 device and comparing the outcomes with whole-body DXA measurements obtained using the Hologic device. The investigation revealed robust correlation coefficients of 0.95 or higher across all extremities, indicating a strong linear relationship between the two methods, as shown in Table 2 and Figure 1. To gauge disparities, muscle mass readings obtained from the Hologic device were subtracted from those derived from the Inbody 970. These discrepancies were then presented as average values for both the left and right arms and for legs, including the maximum and minimum limit values (±1.96 SD). These values were visualized using Bland-Altman plots tailored to each distinct site (Fig. 2). The analysis demonstrated that a substantial proportion of the observed values fell within the ±1.96 SD range from the mean, highlighting the minimal differences between the two techniques.

Regarding the comparison between the InBody and GE Lunar measurements, the assessment of muscle mass involved comparing the BIA results obtained from the InBody 970 device with whole-body DXA measurements acquired using the GE Lunar device. The results indicated strong correlation coefficients of 0.93 or higher for most measurements, except for the right arm, which exhibited a correlation coefficient of 0.83, likely influenced by outliers (Table 3, Fig. 3). This underlines the substantial linear relationship between the Inbody and GE Lunar measurements. To quantify divergence, the differences in muscle mass were computed by subtracting the InBody 970 measurements from those obtained with the GE Lunar device. These divergences were then presented as average values for both the left and right arms and both legs, along with the maximum and minimum limit values (±1.96 SD). As in the previous phase, these findings were complemented by Bland-Altman plots tailored for each measurement site (Fig. 4). Similarly, a notable proportion of the results were within the ±1.96 SD range from the mean.

Additionally, the study summarized the muscle mass conversion formulas for different limb segments in Table 4 for the GE Lunar and Inbody 970 devices, and in Table 5 for the Hologic and InBody 970 devices.

The principal findings of this study were that the correlation between the muscle mass measured by the Hologic device and the GE Lunar device and the muscle mass measured by the Inbody 970 device was high and suggested a conversion formula that is important for the evaluation of sarcopenia between the mutual devices. To date, several cross-calibration studies on BIA and DXA have been reported.[27,28]

Yi et al. [21] conducted an analysis of the correlation between body composition measurements obtained from BIA (InBody 970) and GE Lunar equipment, particularly focusing on the agreement of appendicular lean mass, which showed a correlation of 0.95 or higher. These findings are consistent with our results, and in our study, we extended the analysis to include Hologic equipment. In addition, we provide conversion formulas that can be used in clinical settings. Analysis of skeletal muscle mass (whole-body or appendicular) plays an important role in evaluating patients for sarcopenia diagnosis, especially with the increasing global aging population. Although DXA is considered the gold standard for muscle mass measurement and shows a high correlation with magnetic resonance imaging (MRI), BIA is preferred owing to its non-invasiveness, cost-effectiveness, and comparability to more sophisticated imaging techniques such as computed tomography and MRI. However, it is crucial to note that BIA’s accuracy can be influenced by factors such as hydration status, recent food intake, and electrolyte balance, which can lead to variability in measurements. Also, the original single-frequency BIA had measurement errors related to body water status and BMI. To address these accuracy issues, BIA was later developed into MF-BIA and further expanded to direct segmental multi-BIA (DSM-BIA), enabling more precise measurements of each body part. MF-BIA also overcomes the accuracy issues by analyzing intracellular fluid impedance more accurately.

Nevertheless, measurements obtained through MF-BIA have shown varying degrees of accuracy compared with DXA or MRI, and different studies have reported differences in the rate of sarcopenia diagnosis. Several previous studies have evaluated the correlation between MF-BIA and DXA for assessing muscle mass, with some variation in the results depending on the measured portion (lean body mass, appendicular lean mass).[29-31] However, Fang et al. [31] demonstrated a strong correlation between MF-BIA and DXA in estimating muscle mass (r=0.969, P<0.001) among elderly individuals. Similarly, Jeon et al. [32] found a strong correlation between MF-BIA and DXA for muscle mass estimation (R²=0.914-0.917). Meier et al. [30] reported a correlation of 0.86 between DSM-BIA and DXA for muscle mass estimation after adjusting for age and sex. In the current study, we observed similar results in the same category, and we believe that MF-BIA could be useful in the management and diagnosis of sarcopenia.[30] Despite these correlations, the potential variability introduced by external factors like hydration status underscores the need for careful consideration when interpreting BIA results.

The research presented here encounters several constraints. Notably, there is a dearth of comparative studies on these devices, making it challenging to apply the study’s outcomes universally. However, the high correlation coefficients suggest that BIA, particularly the InBody 970, can be a reliable and more accessible alternative to DXA for muscle mass assessment. This is particularly important in settings where DXA may not be readily available or feasible. Furthermore, the conversion formula enables the standardization of muscle mass measurements across different devices, facilitating the comparison of results from various studies and clinical settings. This standardization can lead to more consistent diagnoses and monitoring of sarcopenia and other musculoskeletal conditions. To broaden the implications of these findings across different ethnicities, it is essential to conduct further research, particularly involving Caucasian subjects. Moreover, the study utilized a limited number of participants, which may impact the robustness and generalizability of the results. Future studies should aim to include larger, more diverse populations to validate these findings further and explore the impact of various physiological and external factors on BIA accuracy.

In conclusion, limb muscle mass measurements using Hologic and GE Lunar whole-body DXA and Inbody 970 BIA demonstrated a very high level of concordance. In addition, a conversion formula that bridges limb muscle mass measurements from two widely used whole-body DXA machines and a BIA machine will facilitate sarcopenia research and patient management.