Bromide dilution in adults: optimal equilibration time after oral administration

Extracellular water is often estimated by bromide dilution. The time interval between oral administration of the dose and blood sampling varies considerably among studies. Because at least two processes are involved during equilibration (distribution of the dose and bromide leaving the body), we performed a bromide-dilution experiment and analyzed the decay data by fitting a biexponential curve. The experiment was carried out in 11 healthy adults (8 women and 3 men, age 21-39 yr). Bromide ingestion (60 mg Br/l estimated total body water) was followed by blood sampling at 2, 3, 4, 6, 10, 25, and 105 (or 170) h. The bromide concentration in serum ultrafiltrate was determined with high-performance liquid chromatography. The bromide concentration in serum after dose intake showed a significant decrease over time. One-exponent functions significantly fitted the data. The two-exponent functions fitted the data significantly better (F-test, P < 0.0001). A model combining the data from all subjects fitted just as well. Equilibration time can be defined as the time at which the first exponential function, which represents distribution of the dose, has decreased to 5% of its starting value. According to this model, the equilibration time of orally administered bromide should be 8.5-11.7 h. This can be achieved by an overnight equilibration.