Measure plasma osmolality

Hypotonic (Posm <280) most common scenario;

true excess of free H2O relative to Na

Isotonic (Posm 280–295):

rare lab artifact from

• hyperlipidemia or
• hyperproteinemia

Hypertonic (Posm >295):

excess of another effective osmole (eg, glucose, mannitol) that draws H2O intravascularly;

• for each 100 mg/dL ↑ glc >100 mg/dL → ↓ [Na] by ~2 mEq/L