Calculated plasma osmolality (Posm = 2×Na + glucose/18 + BUN/2.8, + ethanol/4.6), the osmolar gap, and a mg/dL ↔ mmol/L converter — with educational interpretations.
| Cause | Notes |
|---|---|
| Ethanol | The most common cause; include it in the calculation (ethanol/4.6) when measured. |
| Methanol | Metabolizes to formic acid; vision-threatening emergency. |
| Ethylene glycol | Metabolizes to glycolic and oxalic acids; kidney-threatening emergency. |
| Isopropanol | Metabolizes to acetone; osmolal gap without metabolic acidosis. |
| Mannitol | Hyperosmolar therapy raises measured osmolality. |
| Propylene glycol | IV drug diluent (e.g. lorazepam infusions). |
| IV contrast / solvents | Unmeasured osmoles from contrast media or organic solvents. |
| Ketoacidosis / lactic acidosis | Ketones and lactate are not in the formula. |
| Renal failure | Retained unmeasured solutes widen the gap. |
A gap of 10 mOsm/kg or more is a screening clue only. Combine with anion gap, blood pH, ketones, and specific toxic-alcohol testing — interpreted by a clinician.
Exact molecular weights: glucose 18.016, urea nitrogen 2.8014. The Posm formula uses the teaching divisors 18 and 2.8.
Calculated plasma osmolality = 2 × Na (mEq/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8. If ethanol was measured, add ethanol (mg/dL)/4.6. Example: Na 140, glucose 90 mg/dL, BUN 14 mg/dL → 2×140 + 90/18 + 14/2.8 = 290.0 mOsm/kg. With all values in mmol/L, the equivalent form is simply 2 × Na + glucose + BUN. The dividers 18 and 2.8 convert mg/dL to mmol/L.
The usual reference range is 275–295 mOsm/kg. Values below 275 suggest hypo-osmolality (for example dilution from excess water intake or low sodium), and values above 295 suggest hyper-osmolality (for example dehydration, very high glucose, or unmeasured solutes). Reference ranges vary by laboratory, so always compare against your own lab's range.
The osmolar gap is measured osmolality minus calculated osmolality, in mOsm/kg. Example: measured 285 mOsm/kg with calculated 290.0 mOsm/kg → gap = -5.0 mOsm/kg. A gap under 10 mOsm/kg is usual; a gap of 10 or more suggests unmeasured osmoles — classically ethanol or the toxic alcohols methanol, ethylene glycol, or isopropanol. It is a screening clue, not a confirmation.
A gap of 10 mOsm/kg or more means solutes are present that the formula does not count. Causes include methanol, ethylene glycol, isopropanol (acetone), ethanol (the most common), mannitol therapy, propylene glycol (IV drug diluent), IV contrast, alcoholic or diabetic ketoacidosis, lactic acidosis, and renal failure. Example: calculated 294.4 mOsm/kg with measured 330 → gap 35.6 mOsm/kg, an elevated pattern needing clinical correlation.
If a serum ethanol level was measured, including it (ethanol mg/dL / 4.6) prevents mistaking ordinary alcohol intake for a toxic-alcohol pattern. Ethanol 100 mg/dL contributes about 21.7 mOsm/kg. Example: with ethanol 100 mg/dL the calculation gives 310.9 mOsm/kg, and measured 320 gives a gap of 9.1 mOsm/kg — within the usual band. Note: ethanol does not behave as an ideal osmole, and empirical studies suggest a divisor closer to 3.7; this calculator uses the traditional 4.6.
Osmolality is solute per kilogram of water (mOsm/kg); osmolarity is solute per liter of solution (mOsm/L). Because plasma is about 93% water, the two numbers differ by roughly 7%, but in clinical practice the calculated value is reported as mOsm/kg and treated as equivalent for bedside purposes. Measured osmolality is determined by freezing-point or vapor-pressure depression.
Glucose: mg/dL ÷ 18.016 = mmol/L (exact molecular weight 180.156); the bedside formula uses the teaching divisor 18. BUN (blood urea nitrogen): mg/dL ÷ 2.8014 = mmol/L (two nitrogen atoms, 2 × 14.007); the bedside formula uses 2.8. This calculator accepts either unit for glucose, BUN, and ethanol. Sodium is entered in mEq/L, which equals mmol/L for Na⁺.
A negative gap simply means the calculated value exceeds the measured one — for example calculated 337.6 mOsm/kg with measured 335 gives a gap of -2.6 mOsm/kg. Most measured gaps fall between −10 and +10. A negative gap is usually measurement variation, not a clinical finding on its own, and the same < 10 / ≥ 10 interpretation applies.
No. An early presenter has not yet metabolized the alcohol to acid but the gap is already high; a late presenter may have metabolized most of it, so the gap has fallen back while the danger remains. A patient's baseline gap can also sit in the negative range, hiding a real ingestion inside an apparently 'normal' gap. The gap is a screening tool that must be combined with anion gap, blood pH, ketones, and specific toxic-alcohol testing — and interpreted by a clinician.
No. It is an educational calculator: it does the arithmetic (plasma osmolality from Na, glucose, BUN, and optionally ethanol, plus the osmolar gap) and shows textbook-style interpretations. It cannot diagnose poisoning, ketoacidosis, kidney failure, or any fluid disorder, and it does not replace clinical judgment, a physical exam, or your lab's own reference ranges. Never disregard professional medical advice or delay seeking it because of a number on this page — discuss your results with your doctor.
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