About

Serum osmolality measures the concentration of chemicals in the liquid part of the blood. It is a vital parameter in diagnosing hydration status, seizure risks related to hyponatremia, and disorders involving Antidiuretic Hormone (ADH). This clinical decision support tool allows medical professionals to compute the calculated osmolality based on Sodium, Glucose, and Blood Urea Nitrogen (BUN) levels. Crucially, it facilitates the detection of an "Osmolar Gap"-the difference between measured and calculated osmolality.

A significant osmolar gap often indicates the presence of unmeasured osmotically active particles, such as toxic alcohols (methanol, ethylene glycol) or propylene glycol. Accurate calculation requires precise handling of units; this tool seamlessly handles conversion between US units (mg/dL) and standard International (SI) units (mmol/L) to prevent calculation errors in critical care settings.

Formulas

The formula varies depending on the units used for Glucose and BUN. Sodium is always calculated in mEq/L or mmol/L (numerically equivalent).

For US Units (mg/dL):

Osm_calc = 2×Na⁺ + Glucose18 + BUN2.8

For SI Units (mmol/L):

Osm_calc = 2×Na⁺ + Glucose + BUN

Osmolar Gap:

Gap = Osm_measured − Osm_calc

Reference Data

Parameter	Normal Range (US)	Normal Range (SI)	Clinical Significance
Serum Osmolality	275 - 295 mOsm/kg	275 - 295 mmol/kg	Main determinant of water distribution
Osmolar Gap	< 10 mOsm/kg	< 10 mmol/kg	Elevated gap suggests toxic ingestion
Sodium (Na⁺)	135 - 145 mEq/L	135 - 145 mmol/L	Primary extracellular cation
Glucose	70 - 100 mg/dL	3.9 - 5.6 mmol/L	Hyperglycemia raises osmolality
BUN	7 - 20 mg/dL	2.5 - 7.1 mmol/L	Renal function indicator

Frequently Asked Questions

A gap greater than 10 mOsm/kg is considered elevated. This suggests the presence of unmeasured osmotically active substances in the blood, such as ethanol, methanol, ethylene glycol (antifreeze), acetone, or mannitol.

Yes. Ethanol is an osmotically active substance that is not included in the standard calculation formula. If a patient has consumed alcohol, measuring the blood alcohol level is necessary to correct the gap, or one must expect an elevated gap proportional to the intoxication level.

Sodium (Na+) is the primary cation in the extracellular fluid. To maintain electrical neutrality, it is accompanied by anions (primarily Chloride and Bicarbonate). Multiplying Sodium by 2 accounts for these associated anions without measuring them individually.

Measured osmolality is indicated when you suspect toxic alcohol ingestion, in cases of unexplained metabolic acidosis, or when evaluating hyponatremia to differentiate between true hyponatremia and pseudohyponatremia.