About This Serum Osmolality & Osmolar Gap Calculator
The Serum Osmolality & Osmolar Gap Calculator estimates serum osmolality from the major measured osmoles — sodium, glucose and urea (BUN) — with an optional term for measured ethanol. When a laboratory-measured osmolality is also entered, the tool computes the osmolar gap (measured minus calculated). A widened gap suggests the presence of unmeasured osmoles and, in the right clinical context, is an important early clue to toxic-alcohol poisoning (methanol, ethylene glycol) and other low-molecular-weight substances such as isopropanol, acetone, mannitol or propylene glycol.
This calculator uses conventional (US) units: sodium in mEq/L and glucose, BUN and ethanol in mg/dL. An SI-unit toggle is a planned future enhancement and is intentionally not built here, because hard-coding mixed unit conversions is a common source of error. In SI units the calculated osmolality is computed differently: Calculated osmolality (mOsm/kg) = 2 × Na (mmol/L) + glucose (mmol/L) + urea (mmol/L) [+ ethanol (mmol/L)], where each solute is already in mmol/L and is added directly (no division). Note that urea (mmol/L) = BUN (mg/dL) ÷ 2.8.
The Formula Explained
The most widely used US formula sums twice the sodium (to account for accompanying anions) plus glucose and urea converted from mg/dL to mmol/L. The optional ethanol term uses the Purssell coefficient of 3.7.
$$ \text{Calc Osm} = 2 \times \text{Na} + \frac{\text{glucose}}{18} + \frac{\text{BUN}}{2.8} \; \left[ + \frac{\text{ethanol}}{3.7} \right] $$
$$ \text{Osmolar Gap} = \text{Measured Osmolality} – \text{Calculated Osmolality} $$
Sodium is in mEq/L; glucose, BUN and ethanol are in mg/dL. The divisors convert mg/dL to mmol/L using each substance’s molecular weight (glucose 180, urea-nitrogen 2.8, ethanol 3.7). The ethanol term is included only when a measured ethanol value is entered.
Clinical Interpretation & Limitations
Calculated osmolality is normally about 275–295 mOsm/kg (values below 275 are low and above 295 are high). The osmolar gap is interpreted as follows:
- Gap < 10 (commonly cited normal range): within the usual reference range, but this does not exclude a toxic alcohol — the range is wide and a small or late ingestion can sit here.
- Gap 10–20 (mildly elevated): non-specific. Causes include early or late toxic-alcohol poisoning, ketoacidosis, lactic acidosis, renal failure, ethanol, mannitol and propylene glycol.
- Gap > 20–25 (significant): strongly suggests methanol or ethylene glycol, especially with a high anion gap metabolic acidosis — consider empiric fomepizole and toxicology consultation. A gap > 50 is almost always a toxic alcohol, mannitol or propylene glycol.
A classic pattern is a high osmolar gap combined with a high anion gap metabolic acidosis; early in toxic-alcohol poisoning the osmolar gap is elevated with a normal anion gap, and this reverses as the alcohol is metabolized.
Limitations
- A normal osmolar gap does not exclude toxic-alcohol poisoning — sensitivity is poor. If clinical suspicion is high, treat (fomepizole) and consult toxicology or poison control rather than waiting for the gap.
- Screening, not diagnosis: definitive diagnosis requires direct serum concentrations of the suspected alcohol.
- Formula-dependence: different published formulas yield slightly different calculated values; keep the formula and the measurement method consistent.
- Measurement method matters: a vapor-pressure osmometer does not capture volatile alcohols and will underestimate the gap. Use freezing-point depression osmometry.
- Ethanol coefficient: 3.7 is the standard divisor; using a larger coefficient under-corrects for ethanol and can leave a falsely elevated residual gap.
- Pseudohyponatremia: severe hyperlipidemia or hyperproteinemia can distort the measured sodium and the calculated osmolality.
Frequently Asked Questions (FAQ)
1. What does the osmolar gap detect?
The difference between measured and calculated osmolality reflects unmeasured osmoles — methanol, ethylene glycol, isopropanol, acetone, mannitol or propylene glycol. A large gap raises concern for one of these, but it cannot tell you which one is present.
2. If the gap is under 10, is the patient in the clear?
Only partly. The normal range is wide and an individual baseline is usually unknown, so early or late presentations can have a normal gap. A normal osmolar gap must not be used on its own to rule out a toxic alcohol.
3. Why is an ethanol term included?
Ethanol is the most common cause of an elevated osmolar gap. If you do not account for it, its contribution inflates the gap; subtracting the ethanol term (ethanol ÷ 3.7) means any residual gap better reflects more dangerous osmoles.
4. What is the difference between osmolality and osmolarity?
The calculated value approximates osmolarity (mOsm/L), while the laboratory measures osmolality (mOsm/kg) by freezing-point depression. By convention the osmolar gap is measured osmolality minus calculated osmolarity, with the units treated as interchangeable for this estimate.
5. Does a high gap confirm methanol or ethylene glycol poisoning?
No. A high gap raises suspicion and, with a high anion gap acidosis, may justify empiric fomepizole and toxicology consultation, but confirmation requires direct serum drug concentrations.
⚠️ Disclaimer:
This tool is for informational and educational purposes only and is not a substitute for professional clinical judgment. All treatment decisions must be made by a qualified healthcare professional considering the individual patient’s full clinical context. A normal osmolar gap does not exclude toxic-alcohol poisoning.
