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

Figures & data

Table 1. Baseline demographics and clinical characteristics of enrolled patients.

Characteristic	All patients N = 140	ALS N = 77	PMA N = 33	PLS N = 30
Sex, male	89 (63.6%)	51 (66.2%)	23 (69.7%)	15 (50.0%)
Age at enrollment, years	62.0 ± 10.3	59.9 ± 10.2	65.3 ± 9.1	63.8 ± 11.0
Symptom onset, spinal	113 (80.7%)	58 (75.3%)	32 (97.0%)	23 (76.7%)
Symptom duration, months*	26.7 (15.7–68.2)	17.0 (12.9–27.8)	43.3 (24.5–82.7)	115.5 (59.4–177.0)
ALSFRS-R score	38.2 (5.2)	38.9 (5.0)	38.1 (5.3)	36.3 (5.3)
ΔFRS*	0.23 (0.12–058)	0.42 (0.20–0.79)	0.19 (0.08–0.44)	0.09 (0.06–0.17)
FVC, %	93.3 (19.1)	94.1 (18.5)	88.5 (22.4)	96.5 (16.3)
Riluzole use	94 (67.1)	64 (83.1)	24 (72.7)	6 (20.0)
BMI, kg/m^2*	25.5 (4.6)	25.1 (4.8)	25.7 (5.2)	26.0 (3.8)
Waist circumference, cm	94.4 (11.9)	93.7 (11.2)	98.1 (12.0)	92.1 (13.1)
Atrophy score, right-sided*	2.0 (1.0–4.0)	3.0 (1.0–4.0)	3.5 (2.0–4.0)	1.0 (0.0–2.0)
PG-SGA category
Well nourished	99 (70.7%)	50 (64.9%)	24 (72.7%)	25 (83.3%)
Moderately malnourished	36 (25.7%)	25 (32.5%)	7 (21.2%)	4 (13.3%)
Severely malnourished	2 (1.4%)	0 (0.0%)	2 (6.1%)	0 (0.0%)

Note: Data are represented as mean (SD).

* Data are median values with their 25–75% interquartile range.

Abbreviations: ALS: amyotrophic lateral sclerosis; PMA: progressive muscular atrophy; PLS: primary lateral sclerosis; ALSFRS-R: revised amyotrophic lateral sclerosis functional rating scale; ΔFRS: progression rate determined by: (48 - ALSFRS-R total score) / disease duration); FVC: forced vital capacity; BMI: body mass index; PG-SGA: patient-generated subjective global assessment.

Figure 1. Correlation between fat-free mass obtained by ADP versus BIA and GAL. Scatterplot of the FFM (in kg) values obtained from the (A) BIA and (B) GAL, both compared to ADP. BIA outcomes were used in Kyle’s equation (30). BIA: bioelectrical impedance analysis; ADP: air-displacement plethysmography; r: Pearson’s r correlation coefficient; FFM: fat-free mass; GAL: Gallagher formula.

Figure 2. Differences in FFM when determined by BIA, GAL, and ADP. Bland–Altman plots visualizing the differences in FFM (in kg) of the BIA (panel A) and GAL (panel B) compared to ADP in MND patients (n = 140). Blue area indicates the 95% confidence intervals around the mean difference. Dotted lines indicate the 95% limits of agreement. BIA outcomes were used in Kyle’s equation (30). BIA: bioelectrical impedance analysis; ADP: air-displacement plethysmography; ICC: intraclass correlation coefficient; CI: confidence interval; SEM: standard error of measurement; FFM: fat-free mass; GAL: Gallagher formula.

Table 2. Overview of the measured FFM and FM (kg and %) and the difference between the BIA, GAL and ADP in patients with MND (n = 140).

Method	FFM (kg)	FFM (%)	FM (kg)	FM (%)
ADP	51.7 ± 0.9	65.8 ± 0.8	27.2 ± 0.8	34.2 ± 0.8
BIA	54.2 ± 1.0	68.6 ± 0.7	24.8 ± 0.7	31.4 ± 0.7
Difference BIA versus ADP (95% CI)	2.5 (1.8–3.2)	3.0 (2.1–3.8)	−2.4 (−3.2 to −1.8)	−2.8 (−3.8 to −2.1)
p-value	<0.001	<0.001	<0.001	<0.001
GAL	56.6 ± 0.9	72.0 ± 0.7	22.3 ± 0.7	28.0 ± 0.7
Difference GAL versus ADP (95% CI)	4.9 (4.2–5.7)	6.2 (5.3–7.1)	−4.9 (−5.7 to −4.2)	−6.2 (−7.1 to −5.3)
p-value	< 0.001	< 0.001	< 0.001	< 0.001

Note: Data are represented as mean ± SE. BIA outcomes were used in Kyle’s equation (30) to determine FFM.

Abbreviations: FFM: fat-free mass; FM: fat mass; ADP: air-displacement plethysmography; BIA: bioelectrical impedance; CI: confidence interval; GAL: Gallagher formula.

Figure 3. Differences in calculating predicted REE and MI using data provided by BIA and ADP. Figure shows probability densities of (A) the predicted REE based on data provided by BIA versus ADP, and (B) having a certain metabolic index based on data provided by BIA versus ADP, in patients with MND. BIA outcomes were used in Kyle’s equation (30). The horizontal bars below every panel provide the median (black dots) with their 25–75% interquartile range. pREE: predicted resting energy expenditure; BIA: bioelectrical impedance; ADP: air-displacement plethysmography; MI: metabolic index. P-values are Wilcoxon signed-rank test.

Kyle UG, Genton L, Karsegard L, Slosman DO, Pichard C. Single prediction equation for bioelectrical impedance analysis in adults aged 20-94 years. Nutrition. 2001;17:248–53.

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Data availability statement

All protocol, analyses, and anonymized data will be shared on request from any qualified investigator. We take full responsibility for the data, the analyses and interpretation, and the conduct of the research.