Bedside quantification of fat-free mass in acute spinal cord injury using bioelectrical impedance analysis: A psychometric study

Study Design: Psychometric. Objectives: Assess the validity of bioimpedance-based measures of fat-free mass (FFM) in acute SCI and of current definitions of obesity based on body mass index (BMI). Setting: Australia. Methods: All admissions within eight weeks of a new traumatic SCI were screened. 29% were eligible. 71% of those consented. Twenty participants (18 male) completed deuterium dilution (DD) and bioimpedance-based measurements of FFM. Thirteen also underwent dual-energy X-ray absorptiometry. Strength of relationships and agreement were examined using Lin's concordance coefficient and limits of agreement analysis, respectively. Sensitivity and specificity were calculated for three BMI cutoffs for obesity, using percentage fat mass (%FM) obtained from DD as reference. Results: Median time since injury was 41 days (IQR 28-48). FFM from DD and DXA were highly correlated but not identical. Concordance and agreement between DD and seven bioimpedance-based predictive equations are presented. The best-fitting equation demonstrated a low bias (+0.6 kg) and moderate dispersion (±5.2 kg). The cutoff for overweight in able-bodied people (BMI ≥25 kg/m2) provided sensitivity of 43.8%, compared to 25% for the cut-off for obesity (BMI ≥30 kg/m2). FM from bioimpedance gave the highest sensitivity (88.9%). Conclusions: BMI demonstrates poor specificity to classify obesity in acute SCI. Present findings support the utility of bioimpedance-based measurements for estimating FFM in acute SCI for group comparisons. These results are generalizable to traumatic SCI 4-8 weeks post injury; however, the present data reflect a high proportion of high cervical injuries. Further research is indicated to establish validity for assessment of individuals and for longitudinal monitoring.