Extracellular Volume Estimation from Ratios of Bromide to Chloride in Urine or Saliva

Abstract

Abstract

Use of either urine or saliva samples to estimate extracellular water volume was investigated in 10 men using nonradioactive bromide (Br) and in seven newborn piglets using radioactive Br (⁸²Br) and chloride (³⁶Cl). The relation of Br to Cl concentrations in urine enabled an estimation of Br dilution volume from human urine (267 ± 42 ml/kg, mean ± SD) that was not significantly different (P = 1.0) from the Br dilution volume calculated from plasma Br concentration (268 ± 20 ml/kg). Although the Br dilution volume estimated from saliva was not different from that of plasma, the error in the estimates of Br dilution volume from saliva was too large (mean difference, −36 ± 64 ml/kg) to make its use practical. The data from piglets showed good agreement between ⁸²Br and ³⁶Cl dilution volumes calculated from 4-hr plasma samples (356 ± 14 ml/kg and 347 ± 12 ml/kg; P > 0.1) and between ⁸²Br dilution volumes calculated from urine ⁸²Br:³⁶Cl and plasma ⁸²Br (360 ± 31 ml/kg and 356 ± 14 ml/kg; P > 0.1). Extracellular water volume can be estimated in both adult and young animals using the Br dilution volume calculated from urine samples. It requires (i) two urine collections: one before and one 4 to 8 hr after administration of Br; (ii) a measurement or estimate of plasma Cl concentration; and (iii) a correction factor that describes the relationship of the ratio of Br to Cl in urine to that ratio in plasma.