A comparison between bioelectrical impedance analysis and air-displacement plethysmography in assessing fat-free mass in patients with motor neurone diseases: a cross-sectional study

To determine the validity of bioelectrical impedance analysis (BIA) in quantifying fat-free mass (FFM) compared to air-displacement plethysmography (ADP) in patients with a motor neurone disease (MND). FFM of 140 patients diagnosed with MND was determined by ADP using the BodPod (i.e. the gold standard), and by BIA using the whole-body Bodystat. FFM values were translated to predicted resting energy expenditure (REE); the actual REE was measured using indirect calorimetry, resulting in a metabolic index. Validity of the BIA compared to the ADP was assessed using Bland-Altman analysis and Pearson’s <i>r</i>. To assess the clinical relevance of differences, we evaluated changes in metabolic index and in individualized protein demand. Despite the high correlation between ADP and BIA (<i>r =</i> 0.93), averaged across patients, the assessed mean fat-free mass was 51.7 kg (± 0.9) using ADP and 54.2 kg (± 1.0) using BIA. Hence, BIA overestimated fat-free mass by 2.5 kg (95% CI 1.8–3.2, <i>p</i> &lt; 0.001). Clinically, an increased metabolic index would be more often underdiagnosed in patients with MND using BIA (31.4% according to BIA versus 44.2% according to ADP, <i>p</i> = 0.048). A clinically relevant overestimation of ≥ 15 g in protein demand was observed for 4 (2.9%) patients using BIA. BIA systematically overestimates FFM in patients with MND. Although the differences are limited with ADP, underscoring the utility of BIA for research, overestimation of fat-free mass may have consequences for clinical decision-making, especially when interest lies in determining the metabolic index.

本研究旨在明确生物电阻抗分析法（bioelectrical impedance analysis, BIA）相较于空气置换体积描记法（air-displacement plethysmography, ADP）在量化运动神经元病（motor neurone disease, MND）患者去脂体重（fat-free mass, FFM）中的有效性。本研究纳入140例确诊MND的患者，分别采用作为金标准的BodPod设备开展空气置换体积描记法（ADP）检测，以及采用全身Bodystat设备开展生物电阻抗分析法（BIA）检测，以测定其去脂体重（FFM）。将测得的FFM数值转换为预测静息能量消耗（resting energy expenditure, REE），并通过间接测热法测量实际静息能量消耗，进而计算代谢指数。采用布兰德-奥特曼分析与皮尔逊相关系数r评估BIA相较于ADP的有效性。为评估差值的临床相关性，本研究分析了代谢指数与个体化蛋白质需求量的变化情况。尽管ADP与BIA的相关性较高（r=0.93），但所有患者的平均FFM测定值分别为：ADP法51.7kg（±0.9），BIA法54.2kg（±1.0）。因此，BIA法对FFM的高估幅度达2.5kg（95%置信区间1.8~3.2，p<0.001）。临床层面，采用BIA法评估时，代谢指数升高的情况更易被漏诊（按BIA法计算占比31.4%，按ADP法为44.2%，p=0.048）。采用BIA法时，有4例（2.9%）患者的蛋白质需求量被高估≥15g，具有临床相关性。本研究表明，BIA法会系统性高估MND患者的去脂体重。尽管与ADP法的差值有限，提示BIA法仍可用于相关研究，但去脂体重的高估可能会影响临床决策，尤其当研究关注代谢指数的测定时。