Study of the fat-free mass obtained by two different body composition methods in patients with amyotrophic lateral sclerosis: Impedancemetry versus measurement of the four skin folds
Résumé
Rationale: Amyotrophic lateral sclerosis (ALS) is a severe neurological disease very often accompanied by malnutrition. Nutritional status and body composition are factors associated with survival. The total body bioelectric impedance analysis (BIA) with a validated equation (Desport et al.) allows to assess the body composition during this disease, but requires the appropriate equipment. Measuring body composition using the four skin folds is simple to perform but not validated in ALS. The aim of our work was to study the agreement of measurements of Fat-Free Mass (FFM) by BIA compared to the skin folds method.
Methods: We included all the measurements between 2000 and 2018, for which the ALS patients had a BIA and the measurement of the four skin folds. Skin fold measurements (tricipital, bicipital, subscapular and super-iliac) were performed 3 times on each side and then averaged. The Fat Mass (FM) obtained from the formula of Siri and Durnin & Womersley, allowed us to obtain the FFMsf (weight minus FM). The BIA was performed at 50 kHz. The FFMimp in BIA was calculated using the validated equation of Desport et al. also requiring the measurement of the tricipital skin fold. The correlation between the two methods was evaluated by linear regression and the agreement by the Bland-Altman method.
Results: 1045 measurements were studied. The mean age of the patients was 65.0 ± 11.6 years and the sex ration M / F ratio was of 1.1. In BIA, the mean FFMimp was of 46.1 ± 9.1 kg and FFMsf of 44.5 ± 9.3 kg. There was a good correlation between the two methods (R = 0.91, p <0.0001). On the other hand, the agreement limits on the Bland and Altman graph were +5.7 and -8.9 kg, with an average risk of error of 16.4% which is not acceptable in clinical practice. FFMsf underestimated the FFM on average with 1.6 ± 3.7 kg (3.6%) compared to the FFMimp obtained by BIA.
Conclusion: The measurement of body composition in ALS by skin folds method underestimates the FFM compared to the BIA with reference formula. The limits of agreement are important and not acceptable in clinical practice. This suggests that this method, although simple and inexpensive, cannot be used to determine FFM in ALS patients.