Serum tonicity is related to but distinct from the serum total osmolality. Serum total osmolality represents the total number particles with osmotic activity in serum but does not distinguish ineffective osmoles, substances that freely permeate biologic membranes (and thus equilibrate with serum), from effective osmoles, substances that do not freely permeate membranes (1). Serum tonicity (also known as serum effective osmolality) represents the portion of serum osmoles that are not freely permeableand thus can influence the movement of water across cell membranes (2). Serum tonicity is a better clinical measure than total osmolality for estimating the risk for osmotic complications in humans (3).
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