Educational use only: serum osmolality is not a standalone diagnosis
The calculated value and osmolar gap shown by this page are educational estimates. They are not a medical device, treatment recommendation, triage rule, or substitute for a clinician. Toxic alcohol ingestion, severe hypernatremia, profound hyponatremia, diabetic ketoacidosis, and shock can be time-critical emergencies. A clinician must interpret the calculation with the complete history, examination, blood gas, electrolytes, anion gap, renal function, ketones, lactate, toxicology, and local laboratory method.
Serum osmolality calculation at a glance
Conventional formula: 2 x Na + glucose/18 + BUN/2.8, using sodium in mEq/L and glucose and BUN in mg/dL.
SI formula: 2 x Na + glucose + urea, using sodium, glucose, and urea in mmol/L.
Osmolar gap: measured osmolality minus calculated osmolality.
Common alert threshold: a gap above 10 mOsm/kg H2O suggests unmeasured osmoles and needs clinical correlation.
# What serum osmolality measures and why clinicians calculate it
Serum osmolality estimates the concentration of osmotically active particles in plasma water. Sodium salts contribute most of the normal value, which is why the formula doubles sodium: the major extracellular cation is paired with anions such as chloride and bicarbonate. Glucose and urea also contribute, especially when glucose is markedly elevated or renal clearance of urea is impaired. The calculated result is often compared with a laboratory measured osmolality to look for substances that are present in blood but not represented in the formula.The calculation is most useful when a clinician is already evaluating a specific problem: altered mental status, severe dehydration, hypernatremia, hyponatremia, high anion gap metabolic acidosis, suspected alcohol ingestion, or unexplained laboratory discordance. A normal estimated osmolality does not make the patient safe; it simply describes the expected osmotic load from sodium, glucose, and urea.- Osmolality
- Osmoles of solute per kilogram of solvent. Serum osmolality is reported as mOsm/kg H2O.
- Osmolar gap
- Measured osmolality minus calculated osmolality. It approximates unmeasured osmotically active solutes.
- Tonicity
- Effective osmolality that drives water movement across cell membranes; urea raises osmolality but is usually an ineffective osmole.
- BUN
- Blood urea nitrogen, commonly reported in mg/dL in US laboratories.
- Urea
- The full urea molecule, commonly reported in mmol/L in SI laboratory systems.
| Input | Accepted units | Role in formula | Clinical note |
|---|---|---|---|
| Sodium | mEq/L or mmol/L | 2 x Na | Dominant contributor to normal serum osmolality |
| Glucose | mg/dL or mmol/L | glucose/18 or glucose | Large effect in severe hyperglycemia |
| BUN / urea | BUN mg/dL or urea mmol/L | BUN/2.8 or urea | Raises osmolality but contributes less to tonicity |
| Measured osmolality | mOsm/kg H2O | measured minus calculated | Required to calculate osmolar gap |
# How to interpret the osmolar gap
The osmolar gap is the difference between laboratory measured osmolality and the osmolality predicted from routine chemistry values. When the measured value is much higher than expected, the blood contains additional osmoles. These may be clinically benign, iatrogenic, or dangerous. The usual teaching threshold is 10 mOsm/kg H2O, but this is not a universal diagnostic boundary. Laboratory method, timing, ethanol level, renal function, and acid-base status can all change interpretation.Gap below 10
Usually considered within the expected range when the patient is clinically stable and laboratory values are reliable.
- Does not exclude early or late toxic alcohol exposure
- Review anion gap, pH, lactate, ketones, and history
Gap 10 to 20
Borderline or mildly elevated; repeat testing and clinical context decide significance.
- Consider ethanol, ketoacidosis, renal failure, mannitol, propylene glycol
- Check whether measured osmolality was by freezing-point depression
Gap above 20
More concerning for a substantial unmeasured osmole load, especially with acidosis or altered mental status.
- Toxic alcohols become a priority consideration
- Urgent toxicology or poison center input may be appropriate
A falling osmolar gap can be dangerous in toxic alcohol poisoning
Methanol and ethylene glycol initially raise the osmolar gap as parent alcohols. As metabolism proceeds, the parent alcohol concentration can fall while formate or glycolate accumulate, producing severe anion gap metabolic acidosis. This means a patient can look biochemically worse even as the osmolar gap becomes less impressive. Never use a normal or falling gap by itself to rule out poisoning.
2 x Na sodium salt contribution
18 glucose mg/dL to mmol/L divisor
2.8 BUN mg/dL to urea mmol/L divisor
>10 common elevated gap threshold
# Common causes of an elevated osmolar gap
- Methanol: classically associated with visual symptoms and high anion gap metabolic acidosis after metabolism to formate.
- Ethylene glycol: associated with renal injury, calcium oxalate crystals, hypocalcemia, and glycolate-driven acidosis.
- Ethanol and isopropanol: raise the osmolar gap; isopropanol typically causes ketosis without the same high anion gap acidosis pattern.
- Propylene glycol: can accumulate from medication solvents, especially in high-dose infusions or renal dysfunction.
- Mannitol or hyperosmolar therapies: intentionally raise measured osmolality in selected neurocritical care contexts.
- Ketoacidosis and renal failure: can produce smaller or moderate gaps through accumulated organic solutes.
Strengths and limitations of osmolar gap screening
Advantages
- Uses routine chemistry plus one measured osmolality value, so it can be calculated quickly.
- Helps explain discordance between measured osmolality and the expected sodium-glucose-urea load.
- Can support urgent evaluation when paired with high anion gap metabolic acidosis.
- Useful in teaching the difference between osmolality, osmolarity, and tonicity.
Disadvantages
- Sensitivity changes over time after ingestion because parent alcohols are metabolized.
- A mild elevation is nonspecific and may reflect ethanol, ketoacidosis, renal failure, or laboratory variability.
- Cannot identify which unmeasured osmole is present without targeted testing.
- Calculated osmolality formulas vary; different formulas can produce different gap values.
Practical example: suspected toxic alcohol ingestion
A patient arrives confused with sodium 140 mEq/L, glucose 90 mg/dL, BUN 14 mg/dL, measured osmolality 330 mOsm/kg H2O, bicarbonate 10 mmol/L, and an elevated anion gap. The calculated osmolality is about 290 mOsm/kg H2O, giving a gap near 40. That pattern does not diagnose a specific substance, but it is concerning enough to trigger urgent toxic alcohol evaluation, antidote consideration under local protocol, and specialist consultation.
# Unit handling: glucose, BUN, and urea
A frequent source of error is mixing BUN and urea. BUN reports only the nitrogen portion of urea and is usually expressed in mg/dL. Urea in SI reports the whole molecule in mmol/L. The relationship used in this calculator is BUN mg/dL divided by 2.8 equals urea mmol/L. Glucose mg/dL divided by 18 equals glucose mmol/L. Sodium in mEq/L and mmol/L is numerically equivalent for this formula because sodium has a valence of one.| Conversion | Formula | Example |
|---|---|---|
| Glucose mg/dL to mmol/L | mg/dL / 18 | 180 mg/dL = 10 mmol/L |
| Glucose mmol/L to mg/dL | mmol/L x 18 | 5.6 mmol/L = 100.8 mg/dL |
| BUN mg/dL to urea mmol/L | mg/dL / 2.8 | 14 mg/dL = 5 mmol/L |
| Urea mmol/L to BUN mg/dL | mmol/L x 2.8 | 7 mmol/L = 19.6 mg/dL |
Check the lab label before entering nitrogen values
If the report says BUN, use the BUN mg/dL option. If it says urea or reports mmol/L, use the urea mmol/L option. Entering urea as if it were BUN can materially distort the calculated osmolality and the osmolar gap.# Serum osmolality, tonicity, and sodium disorders
Osmolality and tonicity are related but not identical. Urea contributes to measured osmolality, but because it crosses cell membranes relatively freely, it is usually an ineffective osmole for sustained water shifts. Effective tonicity is driven mainly by sodium salts and glucose. This distinction matters in hyponatremia. Hyperglycemia can pull water out of cells and lower measured sodium by dilution; urea may raise osmolality without causing the same transcellular water movement.Use measured osmolality in the hyponatremia workflow
In hypotonic hyponatremia, measured osmolality is typically low. If sodium is low but measured osmolality is normal or high, consider hyperglycemia, mannitol, radiocontrast, ethanol, toxic alcohols, or pseudohyponatremia. Calculated osmolality helps frame the problem, but measured osmolality is the laboratory anchor.
Clinical bottom line
Calculated serum osmolality is a fast estimate; osmolar gap is a clue to unmeasured osmoles. A gap above 10 mOsm/kg H2O deserves attention, but the result must be interpreted with acid-base data, anion gap, renal function, medication exposures, timing, and toxicology resources. This page is educational and intentionally conservative: it flags risk, it does not make diagnoses.