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 (TBCl) and total body potassium (TBK) measurements for asses sing body fluid compartments of children. Objective: This study assessed relations among independent measurements of TBCl, TBK, and total body water (TBW) in children with CF. Design: We compared cross-sectional measurements of TBCl by in vivo neutron activation analysis, TBK by whore-body counting of K-40, TBW by D2O diluti on [TBW(D2O)], and TBW from TBCl and TBK [TBW(Cl + K)] in 19 prepubertal ch ildren (13 boys) aged 7.6-12.5 y who had mild symptoms of CF. Body composit ion measurements were compared with data from previous studies of healthy c hildren. Results: Subjects with CF had deficits in TBCl, TBK, TBW, and body weight c ompared with control reference data (P < 0.05). The ratios (TBCl + TBK)/TBW and TBCl/TBK were not significantly different from control reference value s, and plasma chlorine and potassium concentrations were within control ref erence ranges. The sum of TBCl and TBK correlated with TBW(D2O) (r(2) = 0.7 9, P < 0.001), and TBW(Cl + K) correlated with TBW(D2O) (r(2) = 0.78, P < 0 .001). TBW(Cl + K) was similar to TBW(D2O) ((x) over bar +/- SEM: 19.0 +/- 0.5 compared with 19.4 +/- 0.5 L; NS). Conclusions: Prepubertal children with mild symptoms of CF can develop defi cits in TBCl, TBK, and TBW that reflect chronic energy malnutrition. Mild s ymptoms of CF do not appear to affect normal relations among TBCl, TBK, and TBW, Measurements of TBCl and TBK may be used to assess body fluid compart ments in these patients.