Isotonicity

Isotonicity refers to the property of having the same osmotic pressure as a reference solution, typically body fluids. An isotonic solution has an osmolality roughly equal to that of human plasma, about 275–295 milliosmoles per kilogram. When such a solution is placed in contact with cells, there is no net movement of water across the cell membrane due to osmosis, so cells retain their volume.

In medical practice, isotonic solutions are commonly used for intravenous infusion to restore circulating volume without causing cellular swelling or shrinkage. Common isotonic intravenous fluids include normal saline (0.9% sodium chloride) and lactated Ringer's solution. Hypertonic solutions have higher osmolality and can draw water out of cells; hypotonic solutions have lower osmolality and can cause cells to swell. The choice depends on the clinical context.

In mathematics and related fields, isotonic describes an order-preserving mapping. An isotonic (or monotone increasing) function f satisfies x ≤ y implies f(x) ≤ f(y) for all x, y in its domain. Isotonicity is a key assumption in monotone regression and related statistical methods; isotonic regression yields a monotone nondecreasing fit to data, often computed by the Pool Adjacent Violators Algorithm (PAVA).