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Comparison of total body chlorine, potassium, and water measurements in children with cystic fibrosis^1,2,3

¹ From the Department of Medicine, Monash University; the Department of Medical Physics and the Body Composition Laboratory, Monash Medical Centre; the Department of Gastroenterology, Royal Children's Hospital; and the Department of Child Psychiatry, Austin Hospital, Melbourne.

Background: Symptoms of cystic fibrosis (CF) may limit the utility of total body chlorine (TBCl) and total body potassium (TBK) measurements for assessing 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 whole-body counting of ⁴⁰K, TBW by D₂O dilution [TBW(D₂O)], and TBW from TBCl and TBK [TBW(Cl + 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 TBCl, TBK, TBW, and body weight compared with control reference data (P < 0.05). The ratios (TBCl + TBK)/TBW and TBCl/TBK were not significantly different from control reference values, and plasma chlorine and potassium concentrations were within control reference ranges. The sum of TBCl and TBK correlated with TBW(D₂O) (r² = 0.79, P < 0.001), and TBW(Cl + K) correlated with TBW(D₂O) (r² = 0.78, P < 0.001). TBW(Cl + K) was similar to TBW(D₂O) ( ± 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 TBCl, TBK, and TBW that reflect chronic energy malnutrition. Mild symptoms 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 compartments in these patients.

Key Words: Body composition • total body chlorine • total body potassium • total body water • extracellular water • intracellular water • cystic fibrosis • prepubertal children