Changes in muscle and fat mass with haemodialysis detected by multi-frequency bioelectrical impedance analysis

BACKGROUND/OBJECTIVES: Multi-frequency bioelectrical impedance analysis (MFBIA) is becoming more widely used to assess hydration status and body composition in haemodialysis patients. Most centres only measure MFBIA pre dialysis when patients are overhydrated. We wished to determine whether body composition assessments change post dialysis following fluid removal. SUBJECTS/METHODS: Lean body and fat mass were measured by MFBIA pre and post haemodialysis in 676 stable outpatients. RESULTS: Weight fell post dialysis from 72.9 +/- 17.8 to 70.9 +/- 19.9 kg, P < 0.001, soft lean mass from 48.2 +/- 12.1 to 45.4 +/- 11.0 kg and fat-free mass from 51.8 +/- 19.2 to 48.1 +/- 11.8 kg, P < 0.001, whereas percentage body fat (PBF) increased from 28.8 +/- 11.9 to 30.8 +/- 12.1% post dialysis, P < 0.001, with a mean increase post dialysis of 2.0% (95% confidence limits 1.55 to 2.45). There were correlations between the fall in total body water and extracellular water and skeletal muscle mass (r = 0.826, P < 0.001 and r = 0.711, P < 0.001, respectively), and negative correlation between the fall in total body water and ICW and the increase in PBF (r = -0.72, P < 0.001, and -0.72, P < 0.001, respectively). The relative changes were greater for the arms compared with the legs. CONCLUSIONS: Although more convenient for both patients and staff to undertake bioimpedance measurements pre dialysis, overhydration over estimates muscle mass and under estimates fat. For more reliable and reproducible assessments of nutritional status, we suggest that bioimpedance measurements of body composition should be made when patients are closer to their target weight than when overhydrated.