Serum Osmolality Calculator

The calculation relies on the primary active solutes in the bloodstream: sodium, glucose, and blood urea nitrogen (BUN), with an optional adjustment for ethanol. The standard mathematical formula is expressed as: Osmolality = (2 * Na) + (BUN / 2.8) + (Glucose / 18) + (Alcohol / 3.7). Sodium is multiplied by two because it is accompanied by balancing anions like chloride and bicarbonate. Dividing BUN by 2.8 and glucose by 18 converts milligrams per deciliter into milliosmoles per liter. The osmolar gap is subsequently found by subtracting the calculated osmolality from the measured laboratory osmolality.

Consider a patient with the following laboratory values: Sodium (Na) = 140 mEq/L, Glucose = 108 mg/dL, BUN = 14 mg/dL, and no detectable alcohol. First, multiply the sodium by two: 2 * 140 = 280. Next, divide the glucose by 18: 108 / 18 = 6. Then, divide the BUN by 2.8: 14 / 2.8 = 5. Sum these values together to find the calculated serum osmolality: 280 + 6 + 5 = 291 mOsm/kg. If the measured laboratory osmolality for this patient is 295 mOsm/kg, the resulting osmolar gap is 295 - 291 = 4 mOsm/kg, which falls well within the normal physiological range.

Always ensure your laboratory values use the correct standard units of measurement (mg/dL for glucose and BUN, mEq/L for sodium) before entering them into the calculator to avoid severe numerical errors. When assessing for toxic alcohol ingestion, remember that an elevated osmolar gap strongly suggests the presence of unmeasured low-molecular-weight osmolies such as methanol or ethylene glycol. Finally, always interpret these calculations within the full context of a patient's clinical presentation, vital signs, and arterial blood gas results.