Validation of saliva sampling for total body water determination by H2 18O dilution

The use of saliva sampling for the measurement of total body water by H2 18O stable isotope dilution was assessed. The H2 18O abundance in saliva reached a constant value by three hours after isotope administration, and the total body water calculated from the 3-h saliva values was not significantly different from the value calculated from the 4- and 6-h serum samples. The results indicated that isotopic equilibration between these fluids was rapid and complete. Comparison of total body water calculated from the paired serum and saliva studies indicated that the relative precision of the determination was between 0.8 and 1.5%. Saliva sampling is noninvasive and is well suited for use in children in field studies.