Which electrolyte level correlates best with plasma osmolality?

Sodium is the primary electrolyte that correlates best with plasma osmolality because it is the most abundant cation in extracellular fluid and plays a crucial role in maintaining osmotic balance. Plasma osmolality is largely determined by the concentration of solutes in the blood, with sodium being a key contributor. It influences the movement of water between the intracellular and extracellular compartments, ultimately affecting fluid balance in the body.

The correlation exists because changes in sodium concentration directly impact osmolality; an increase in sodium typically leads to an increase in osmolality and vice versa. While other electrolytes such as chloride and bicarbonate are also present in plasma and contribute to osmolality, sodium's dominant role makes it the best correlate. Calcium, although important for various physiological functions, has a less direct influence on plasma osmolality compared to sodium.