Plasma Osmolality Calculator

Plasma osmolality is a medical measurement that quantifies the total concentration of dissolved particles—including electrolytes, glucose, and proteins—per kilogram of plasma water. It serves as a primary indicator of the body's hydration status, electrolyte balance, and the functional capacity of homeostatic regulatory mechanisms, particularly the kidneys and antidiuretic hormone secretion.

You can calculate plasma osmolality by taking twice the serum sodium concentration and adding it to the values obtained by dividing blood glucose by 18 and blood urea nitrogen (BUN) by 2.8. All input values must be drawn from a concurrent blood sample and expressed in standard units, specifically milligrams per deciliter for glucose and BUN, and milliequivalents per liter for sodium.

The normal reference range for plasma osmolality in healthy adults typically falls between 275 and 295 milliosmoles per kilogram of water (mOsm/kg). Values slightly below this range suggest excess body water or hyponatremia, whereas values above this range point toward dehydration, hypernatremia, or elevated blood solute concentrations like hyperglycemia.

Yes, a plasma osmolality of 300 mOsm/L is slightly elevated above the standard upper limit of 295 mOsm/L. This mild elevation frequently indicates early dehydration, concentrated blood volume, or moderately elevated blood sugar and urea levels. It signals that the body is retaining solutes or lacking adequate free water, warranting a clinical review of fluid intake.