Measuring the size of the extracellular fluid space using bromide, iohexol, and sodium dilution

There is a need to find methods to assess the size of the extracellular fluid (ECF) volume without involving radioactive tracers. For this purpose, we applied 3 methods for measuring the ECF volume in 10 male volunteers (mean age, 34 yr). Steady-state plasma bromide concentration (control) was compared to the results of kinetic analysis of plasma iohexol and to kinetic analysis of the dilution of serum sodium after IV infusion of 1 L of isotonic mannitol. The volume of distribution of these tracers was used to indicate the ECF volume. The results disclosed statistically significant correlations between the results of all 3 methods, although the average sodium dilution showed 0.7 L lower values than iohexol and 1.4 L lower than bromide. All three methods correlated significantly with body weight. The percentage of the body weight indicated by the methods was 18.3% (3.1%) for sodium, 19.6% (1.0%) for iohexol, and 20.5% (1.1%) for bromide. We conclude that sodium dilution may be performed at bedside but iohexol and bromide showed less intersubject variability. Iohexol simultaneously measures the glomerular filtration rate and should be a viable clinical option if the hospital performs routine assessments of kidney function using this tracer.