Comparison of total body chlorine, potassium, and water measurements in children with cystic fibrosis

Background: Symptoms of cystic fibrosis (CF) may limit the utility of total body chlorine (TBCI) and total body potassium (TBK) measurements for assessing body fluid compartments of children. Objective: This study assessed relations among independent measurements of TBCI, TBK, and total body water (TBW) in children with CF. Design: We compared cross-sectional measurements of TBCI by in vivo neutron activation analysis, TBK by whole-body counting of 40 K, TBW by D 2 O dilution [TBW(D 2 O)], and TBW from TBCI and TBK [TBW(C1 + K)] in 19 prepubertal children (13 boys) aged 7.6-12.5 y who had mild symptoms of CF. Body-composition measurements were compared with data from previous studies of healthy children. Results: Subjects with CF had deficits in TBCI, TBK, TBW, and body weight compared with control reference data (P < 0.05 The ratios (TBCI + TBK)/TBW and TBC1/TBK were not significantly different from control reference values, and plasma chlorine and potassium concentrations were within control reference ranges. The sum of TBC1 and TBK correlated with TBW(D 2 O) (r 2 = 0.79, P < 0.001 and TBW(C1 + K) correlated with TBW(D 2 O) (r 2 = 0.78, P < 0.001). TBW(C1 + K) was similar to TBW(D 2 O) (x ± SEM: 19.0 ± 0.5 compared with 19.4 ± 0.5 L; NS). Conclusions: Prepubertal children with mild symptoms of CF can develop deficits in TBCI, TBK, and TBW that reflect chronic energy malnutrition. Mild symptoms of CF do not appear to affect normal relations among TBC1, TBK, and TBW. Measurements of TBCI and TBK may be used to assess body fluid compartments in these patients.