A new method to distinguish the hyponatremia of electrolyte loss from that due to pure solvent changes

Estimates of solute and solvent changes during electrolyte abnormalities are valid only when either total body water (TBW) or solute content do not change, while it cannot be established which one of these is altered. The present paper provides a method capable of distinguishing these two different conditions. When only solvent changes, the respective concentration ratios of plasma (P) solutes PCl/PNa, P/PNa, PCl/POAN (POAN = anions other than Cl) remain unchanged. Moreover, PNa₁/PNa₀ (the ratio of PNa during the derangement over the normal value, indicated by subfix ₁ and ₀, respectively) = Cl₁/PCl₀ = POAN₁/ POAN₀. When these constraints are met, the abnormality is due only to a TBW change, which is easily calculated and corrected. When they are not met, the exact change in Na content is correctly calculated assuming no variation in TBW. These calculations could still be useful even in the presence of TBW modifications, where they represent minimum estimates of electrolyte losses. The formulas were validated by computer simulations generating true electrolyte concentrations, which were then used to back calculate the changes in their contents and extra/ intra-cellular volumes. Since the predicted results were significantly correlated with the true data, the method was transferred to 24 patients with electrolyte disturbances, who met the above constraints. The calculated volume changes were significantly correlated with those obtained by body weight measurements (regression coefficient = 0.94, P < 0.0001), while the quantitative estimates of Na deficit predicted the PNa values measured after corrective treatment (P < 0.0001). This new method may prove valuable in diagnosing and treating electrolyte derangements.