Critical view on diagnosing muscle wasting by single-frequency bio-electrical impedance in COPD

Background: Assessment of muscle wasting in COPD is relevant as it is independently associated with metabolic and functional consequences and even survival. Muscle wasting can be approached by assessing fat free mass (FFM), but it is already demonstrated that FFM measured by bio-electrical impedance analysis (BIA) underestimates FFM measured by dual energy X-ray absorptiometry (FFM(DExA)) in a relatively small COPD group.

Objective: To evaluated critical points for defining muscle wasting in a large cohort of moderate to severe COPD patients and with DEXA scan as reference.

Design: FFM by BIA was compared with FFM(DExA) in 1087 COPD patients (641 male symbol, FEV1: 44.8+/-17.5%pred). In a subgroup (n=422), FFM(DExA) was predicted by multivariate analysis and a new formula to calculate FFM by BIA was developed. The new formula was compared with FFM(DExA) in the remaining subgroup (n=665). Muscle wasting was defined according to the cut-offs of Schols et al. (FFM index (FFMI)<16 kg/m(2) for men, 15 kg/m(2) for women), Vestbo et al. (FFMI<17.1 kg/m(2) for men, 14.6 kg/m(2) for women), and Coin et al. (FFMI<17.8 kg/m(2) for men, 14.6 kg/m(2) for women).

Results: There was an underestimation of FFM by BIA when compared to FFM(DExA) by the Bland Altman. Comparing the new formula with FFM(DExA), the mean underestimation almost disappeared but the variation remained. The proportion of muscle wasting was largely dependent on the used cut-offs, especially in men.

Conclusion: The results of the present study emphasize the importance to accurately bare in mind the technique and cut-offs to establish muscle wasting before implementing it in the clinical practice.