Estimate Serum Osmolality From Sodium, Glucose and BUN

How to Use This Tool

Apply the common mg/dL equation to values from the same blood specimen. Using mmol/L divisors with mg/dL inputs, or mixing specimens, produces a convincing but incorrect estimate.

What Calculated Osmolality actually establishes

Calculated osmolality is not the measured laboratory osmolality and different institutions may use slightly different equations. The page returns calculated osmolality in mosm/kg from the measurements named below; Calculated Osmolality does not verify how they were collected or combine them with symptoms, examination findings and medical history.

Equation and input contract for Calculated Osmolality

Calculated mOsm/kg equals 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8 for the printed units. The required entries are Sodium, Glucose, BUN; values should come from the same dated observation, specimen or record unless a label explicitly calls for different dates.

• For Calculated Osmolality, Sodium uses the example 140 mmol/L; replace it with the value and unit from the same record.
• For Calculated Osmolality, Glucose uses the example 100 mg/dL; replace it with the value and unit from the same record.
• For Calculated Osmolality, BUN uses the example 14 mg/dL; replace it with the value and unit from the same record.

Using calculated osmolality in mosm/kg safely

Use values from the same draw and compare with the laboratory's measured value and equation under clinical supervision. For Calculated Osmolality, an unexpected answer should prompt a check of units, dates, transcription and the original measurement method. Do not adjust medicine, feeding, fluids, exercise, screening or treatment to make the result match an expected value.

The page-specific limit of Calculated Osmolality

Ethanol and other osmoles, unit conversion, laboratory method, water balance diagnosis and treatment are excluded. Preserve that limitation with any saved calculated osmolality in mosm/kg. A reference interval or action threshold still depends on the patient, collection method, laboratory and clinical setting, none of which this arithmetic selects automatically.

Evidence reviewed for Calculated Osmolality

The attached reference is NIH MedlinePlus — Osmolality tests. It supports the equation, definition or measurement practice used for calculated osmolality in mosm/kg, while the original report and a licensed pediatrician, physician, pharmacist or other relevant professional remain authoritative for interpretation.

Emergency boundary

Do not use a calculator during a possible emergency. Severe breathing difficulty, blue or gray color, seizure, fainting, new confusion, stroke symptoms, inability to wake or rapid deterioration require urgent local medical help.

Privacy and repeatability for Calculated Osmolality

Calculated Osmolality runs in the current browser tab without requesting a name, login or medical-record connection. Keep identifying details out of the fields, preserve the original units and remember that extra decimal places cannot repair an uncertain measurement.