Plasma sodium concentration ([Na+]) generally accurately reflects plasma tonicity because sodium and its anions account for almost all the effective osmoles in the extracellular fluid (ECF). Given that the hydraulic permeability of most cell membranes is very high, water transits quickly and freely between the ECF and the intracellular fluid, resulting in equal tonicity in both. In the intracellular fluid, by contrast, potassium and its anions are responsible for the tonicity. On the basis of these principles, Edelman et al1 established in patients studied under steady-state conditions that plasma [Na+] has a predictable relationship with the ratio of total-body “exchangeable” (ie, osmotically active) sodium and potassium content (eNa+ plus eK+) and total-body water (Edelman equation).