Nephrology, more than other specialties is plagued by misplaced concreteness. We get false senses of precision because of the myriad of equations that spit out results to the milliliter. All of those equations from Kt/V, to water deficits, to IVF brain teasers depend on an estimate of total body water.
Everyone knows the rule of thumb that young males are 60% water, young females are 50% water and the percent body water falls as people age or get fatter.
Going beyond these rules of thumb, how is total body water measured empirically? The gold standard is heavy water dilution.
This works by giving a sample of heavy water and then waiting for it to equilibrate. Then the investigators measure the heavy water content of exhaled water vapor or a blood sample, the fraction of the water that is heavy water will be equivalent to the fraction of total water which is heavy water. Then since one knows the amount of heavy water given to the patient, one can calculate total body water.
When this is done, or when one reviews the primary literature, as was done in this study the numbers are a little different.
- Males: TBW (in liters) = 2.447 + (0.09156 × age) + (0.1074 × height) + (0.3362 × weight)
- Females: TBW (in liters) = –2.097 + (0.1069 × height) + (0.3362 × weight)