An Evaluation of the Single Injection Thiosulfate Method for the Measurement of Extracellular Water1

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Abstract
In "hospital normal" women, experimental alteration of the renal clearance rate of thiosulfate, by carinamide or theophylline ethylenediamine, changed significantly the apparent volume of thiosulfate distribution as calculated by the single injection method. Measurements of the thiosulfate space by the infusion/ slope method gave volumes ranging from 0 to 54% (average 30%) greater than the volumes calculated from the single injection method. The rate of plasma clearance of thiosulfate was nearly twice as great following single injection as it was following the prolonged infusion used in the infusion/slope method. This suggests that the time encompassed by the single injection procedure is not long enough for the attainment of equilibrium and that the test does not measure the potential volume of distribution available to thiosulfate. Moreover, the theoretic bases of the method do not seem sound. The apparent volume of thiosulfate distribution as calculated from the single injection method averaged 35% less than the sucrose space, measured almost simultaneously by the calibrated infusion (NV-IV) method. The thiosulfate space, measured by the infusion/slope method, bore no constant relation to the volume of distribution of sucrose, measured simultaneously by the infusion/slope or by the calibrated infusion method. The volumes of distribution of sucrose, measured simultaneously by the infusion/slope and calibrated infusion methods, checked closely. It is concluded that the calculation based upon the single injection method has no significance as a volume and that it only fortuitously falls within a "reasonable" range because of multiple, variable, and partially compensatory errors.