ABSTRACT
OBJECTIVE: Malnutrition is highly prevalent in patients with kidney failure. Since body weight does not reflect body composition, other methods are needed to determine muscle mass, often estimated by fat-free mass (FFM). Bioimpedance spectroscopy (BIS) is frequently used for monitoring body composition in patients with kidney failure. Unfortunately, BIS-derived lean tissue mass (LTMBIS) is not suitable for comparison with FFM cut-off values for the diagnosis of malnutrition, or for calculating dietary protein requirements. Hypothetically, FFM could be derived from BIS data (FFMBIS). This study aims to compare FFMBIS and LTMBIS with computed tomography (CT) derived FFM (FFMCT). Secondarily, we aimed to explore the impact of using different methods on calculated protein requirements. METHODS: CT scans of 60 patients with CKD stage 4-5 were analyzed at the L3 level for muscle cross-sectional area, which was converted to FFMCT. Spearman rank correlation coefficient and 95% limits of agreement (LoA) were calculated to compare FFMBIS and LTMBIS with FFMCT. Dietary protein requirements were determined based on FFMCT, FFMBIS and adjusted body weight. Deviations over 10% were considered clinically relevant. RESULTS: FFMCT correlated most strongly with FFMBIS (r=0.78, p<0.001), in males (r=0.72, p<0.001) and in females (r=0.60, p<0.001). A mean difference of -0.54 kg was found between FFMBIS and FFMCT (LoA: -14.88 to 13.7 kg, p=0.544). Between LTMBIS and FFMCT a mean difference of -12.2 kg was apparent (LoA: -28.7 to 4.2 kg, p<0.001). Using FFMCT as a reference, FFMBIS best predicted protein requirements. The mean difference between protein requirements according to FFMBIS and FFMCT was -0.7 ± 9.9 grams in males and -0.9 ± 10.9 grams in females. CONCLUSION: FFMBIS correlates well with FFMCT at a group level, but still shows large variation within individuals. As expected, large clinically relevant differences were observed in calculated protein requirements.
