Clinical relevance of inter-method differences in fat-free mass estimation in chronic obstructive pulmonary disease

Evaluation of fat-free mass (FFM) is becoming recognised as an important component in the assessment of clinical status and formulation of prognosis in patients with chronic obstructive pulmonary disease (COPD).

The aim of this study was to determine whether potential differences in FFM estimation performed by air displacement plethysmography (ADP), bioelectrical impedance (BIE) and anthropometry (ANTHRO) would assume clinical significance.

Twenty-eight patients with moderate-to-severe COPD were submitted to FFM estimation by ADP, BIE and ANTHRO. FFM was then allometrically related to peak oxygen uptake (peak VO2) as determined by symptom-limited incremental cycle ergometry.

We found that ANTHRO classified fewer patients as 'FFM-depleted' than the other two techniques (p < 0.05). Although mean biases of the BIE-ADP differences were close to zero, their 95% confidence limits extended as high as 5.9 kg (16%). The ANTHRO-based allometric exponents for peak VO2 correction of FFM, therefore, were typically higher than those obtained by the other two methods in both depleted and non-depleted patients (ANTHRO: 1.45-1.41, BIE: 0.97-1.18, ADP: 1.08-1.14, respectively).

We conclude that between-method differences in FFM estimation can be sufficiently large to have practical implications in patients with moderate-to-severe COPD. A single method of body composition assessment, therefore, should be used for FFM estimation in these patients.