see also: [[Blood gas#osmolality and osmolar gap]], [[hyponatremia|hyponatraemia]], [[hypernatremia#Free water deficit / fluid deficit]] see: [path tests - osmolality](https://www.pathologytestsexplained.org.au/ptests-pro.php?q=Osmolality) **osmolality** - laboratory measurement . mOsm/kg . - number of particles dissolved in a kg of fluid - normal range: *275-299* mOsm/kg - very high if >320 ; likely needs ICU if >350 - very low if <240 **osmolarity** - bedside calculation . mOsm/L - = 2Na + urea + Glucose + 1.25 EtOH - number of particles in a litre of fluid - normal serum: *275-299* mOsm/L - normal urine: **Effective Plasma Osmolality** (sometimes erroneously referred to as “tonicity”): > 2xNa + Glucose > (Note that if glucose given in US MG/dL, divide by 18 to get AUD mmol/L) - Urea readily crosses cell membranes, so an increase in blood urea nitrogen (BUN) will not increase the effective osmotic activity of plasma. In other words, azotemia is a hyperosmotic, but *not a hypertonic*, condition. Therefore, the calculation of *effective plasma osmolality* does not include the urea EXAMPLE: Using normal plasma concentrations of Na (140 mEq/L) and glucose (5 mmol/L), the effective plasma osmolality is (2 × 140) + 5 = 285 mosm/kg H2O, which is very close to the total osmolality (290 mosm/kg H2O). ***Significance of sodium:*** - If the glucose is removed from effective osmolality equation, the calculated Posm is 280 mosm/kg H2O, which is 98% of the calculated Posm that includes glucose (i.e., 285 mosm/kg H2O). This demonstrates: 1. Sodium is responsible for 98% of the effective plasma osmolality. 2. The sodium concentration in extracellular fluid (plasma) is the principal factor that determines the distribution of total body water [[hypernatremia#Free water deficit / fluid deficit|total body water]] in the intracellular and extracellular fluid compartments, and hence it is also the principal determinant of the extracellular volume.