Serum Osmolality Calculator
Calculate serum osmolality from sodium, glucose, and BUN values. Enter values for instant results with step-by-step formulas.
Formula
Osmolality = 2 × Na + Glucose/18 + BUN/2.8 + Ethanol/4.6
Where Na = serum sodium in mEq/L, Glucose in mg/dL, BUN (blood urea nitrogen) in mg/dL, and Ethanol in mg/dL. The divisors convert mass concentration to millimolar units.
Worked Examples
Example 1: Normal Patient Assessment
Problem: A patient has sodium 140 mEq/L, glucose 90 mg/dL, and BUN 14 mg/dL. Calculate the serum osmolality.
Solution: Osmolality = 2(140) + 90/18 + 14/2.8\n= 280 + 5.0 + 5.0\n= 290.0 mOsm/kg
Result: Calculated osmolality: 290.0 mOsm/kg — within normal range (275-295)
Example 2: Diabetic Ketoacidosis Patient
Problem: A diabetic patient presents with sodium 132 mEq/L, glucose 450 mg/dL, and BUN 28 mg/dL. Calculate osmolality.
Solution: Osmolality = 2(132) + 450/18 + 28/2.8\n= 264 + 25.0 + 10.0\n= 299.0 mOsm/kg
Result: Calculated osmolality: 299.0 mOsm/kg — hyperosmolar, consistent with DKA
Frequently Asked Questions
What is serum osmolality and why is it important?
Serum osmolality measures the concentration of dissolved particles in the blood, expressed in milliosmoles per kilogram of water (mOsm/kg). It reflects the body's fluid balance and is regulated primarily by antidiuretic hormone (ADH) and thirst mechanisms. Normal serum osmolality ranges from 275 to 295 mOsm/kg. Clinicians use this value to evaluate conditions such as dehydration, overhydration, hyponatremia, hypernatremia, diabetic ketoacidosis, and ingestion of toxic alcohols like methanol or ethylene glycol. Abnormal osmolality values can indicate serious metabolic disturbances requiring immediate medical attention.
How is serum osmolality calculated from lab values?
The standard formula for calculated serum osmolality is: 2 times sodium (mEq/L) plus glucose (mg/dL) divided by 18 plus BUN (mg/dL) divided by 2.8. The sodium is doubled because each sodium ion is accompanied by a corresponding anion such as chloride or bicarbonate. Glucose and BUN are divided by conversion factors to translate their mass concentration into millimolar units. Some formulas also include ethanol divided by 4.6 when alcohol ingestion is suspected. This calculated value is compared against the measured osmolality from the lab to determine the osmol gap, which can reveal unmeasured solutes in the blood.
What is the difference between osmolality and tonicity?
Osmolality measures the total concentration of all dissolved solutes in the blood, including those that can freely cross cell membranes like urea. Tonicity, also called effective osmolality, measures only the solutes that cannot freely cross cell membranes and therefore exert an osmotic force that causes water to shift between compartments. Tonicity is calculated as 2 times sodium plus glucose divided by 18, excluding BUN because urea distributes freely across membranes. Tonicity is more clinically relevant for assessing cellular hydration status and determining the risk of cerebral edema or cellular swelling. Changes in tonicity drive the movement of water in and out of cells.
What conditions cause abnormal serum osmolality values?
Elevated serum osmolality (hyperosmolality above 295 mOsm/kg) can be caused by dehydration, hyperglycemia as in diabetic ketoacidosis or hyperosmolar hyperglycemic state, hypernatremia, uremia, toxic alcohol ingestion, or mannitol administration. Decreased serum osmolality (hypo-osmolality below 275 mOsm/kg) is most commonly caused by excess water intake, syndrome of inappropriate antidiuretic hormone secretion (SIADH), psychogenic polydipsia, or dilutional hyponatremia from intravenous fluid overload. Critical osmolality values below 240 or above 320 mOsm/kg are medical emergencies that can cause seizures, coma, or death if not treated promptly.
How accurate are the results from Serum Osmolality Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.