Osmolar gap: how toxic alcohols hide from the anion gap
The osmolar gap is measured minus calculated osmolality. It picks up uncharged particles such as methanol and ethylene glycol, which raise it early, before the anion gap opens.
Chapters (10)
Calculate the serum osmolar gap (measured − [2 × Na⁺ + glucose/18 + BUN/2.8]) and explain how methanol and ethylene glycol produce an early osmolar gap and a later anion gap as alcohol dehydrogenase converts them to formic acid, and to glycolic and oxalic acid.
The short version
- The osmolar gap is measured minus calculated osmolality, and it is normally below about 10.
- Calculated osmolality in US units is 2 × Na⁺ + glucose/18 + BUN/2.8; in SI units it is 2 × Na⁺ + glucose + urea, all in mmol/L.
- Starting with alcohol dehydrogenase, methanol becomes formic acid, which hits the eyes, and ethylene glycol becomes glycolic and oxalic acid, which hit the kidneys.
- Early the osmolar gap is high; late the anion gap is, so a normal osmolar gap never rules out a toxic alcohol.
- Fomepizole competitively blocks alcohol dehydrogenase so no new acid is made; hemodialysis removes the alcohol and its acids.
The puzzle: a normal anion gap and 35 mOsm/kg missing
An hour after drinking antifreeze, a man's anion gap is still normal. But his measured osmolality is 325, and his labs predict only 290. Thirty-five milliosmoles in every kilogram of water are unaccounted for. That difference is the osmolar gap.
This lesson shows how to calculate it, and why toxic alcohols open the osmolar gap early and the anion gap later.
How to calculate the osmolar gap
Osmolality counts every dissolved particle in a kilogram of water, and the lab measures it directly.[3][8] It can also be estimated from the three main particles in plasma: sodium, glucose and urea.[1][3]
Na⁺ in mEq/L; glucose and BUN in mg/dL. The divisors turn mg/dL into mmol/L (glucose 180 g/mol; BUN counts 2 nitrogen atoms, 28 g/mol).
all in mmol/L
normally below about 10 mOsm/kg
Sources use osmolar gap and osmolal gap interchangeably. A gap above 10 means something osmotically active is in the blood that the formula does not count.[1][3][10]
- Na⁺ 140 mEq/L, glucose 90 mg/dL, BUN 14 mg/dL
- 2 × 140 = 280; 90/18 = 5; 14/2.8 = 5
- Calculated osmolality = 280 + 5 + 5 = 290 mOsm/kg
- Osmolar gap = 325 − 290 = 35 mOsm/kg, far above 10
Osmolar gap vs anion gap: what each one detects
The anion gap reveals unmeasured anions. The osmolar gap reveals unmeasured particles, charged or not.[5][3]
Methanol and ethylene glycol are small molecules with no charge. So when they are swallowed, they pile up as extra particles, raising the osmolar gap but not the anion gap.[3][4]
The real danger is not the alcohol itself. It is what the body makes from it.[4][8]
Toxic alcohols: methanol and ethylene glycol become acids
For both alcohols, the first step is the enzyme alcohol dehydrogenase.[6][7]
| Alcohol | Pathway (first step: alcohol dehydrogenase) | What the acids do |
|---|---|---|
| Methanol | Formaldehyde → formic acid | Formic acid drives the acidosis and injures the retina and optic nerve: blurred vision, which can look like a snowfield, and blindness can follow |
| Ethylene glycol | Glycolaldehyde → glycolic acid → glyoxylic acid → oxalic acid | Glycolic and oxalic acid drive the acidosis; oxalate binds calcium, so calcium oxalate crystals can appear in the urine, with low calcium and kidney injury |
In short: methanol hits the eyes, ethylene glycol hits the kidneys.[6][8][9]
This is why the clinical picture matters in an exam vignette. Both alcohols give the same pair of gaps, so the gaps alone cannot tell them apart. Blurred vision and hyperemic optic discs point to methanol's formic acid; crystals in the urine, low calcium and kidney injury point to ethylene glycol's oxalate.[6][8][9]
Why the osmolar gap falls as the anion gap rises
As the alcohol is converted into acid, the osmolar gap falls and the anion gap rises.[3][9]
This follows from the difference between the two gaps. The parent alcohol has no charge, so it counts only in the osmolar gap. The acids it becomes are what open the anion gap. So as alcohol is turned into acid, one gap shrinks while the other widens, and mid-course both can be open at once.[3][9]
- Early: a big osmolar gap with a normal anion gap.
- Middle: both gaps can be raised at once.
- Late: the anion gap is wide, and the osmolar gap may already be back to normal.
Fomepizole, ethanol and hemodialysis
Because the harm comes from the acids, the antidote works upstream. Fomepizole competitively blocks alcohol dehydrogenase, so no new acid is made.[6][2] Ethanol does the same job by competing for the enzyme; it is the second-line alternative to fomepizole.[7][8]
Both antidotes act on the same first step. Blocking alcohol dehydrogenase stops the alcohol that is still in the blood from being turned into acid.[6][7]
But blocking the enzyme cannot clear what is already there. Severe acidosis or organ damage needs hemodialysis, which removes the alcohol and its acids.[1][7]
The isopropyl alcohol trap
Isopropyl alcohol also raises the osmolar gap. But it becomes acetone, a ketone, not an acid, so it causes ketosis without an anion gap acidosis.[9][4]
Practice question
Original · USMLE styleA 38-year-old man comes to the emergency department because of blurred vision that began this morning. He says it is "like looking through snow". Eighteen hours ago he drank a large amount of home-distilled spirits. He is drowsy but oriented. Pulse is 104/min, respirations are 28/min and blood pressure is 128/78 mm Hg. Fundoscopy shows hyperemic, slightly swollen optic discs. Laboratory studies show:
| Serum Na⁺ | 138 mEq/L |
|---|---|
| Serum K⁺ | 4.2 mEq/L |
| Serum Cl⁻ | 100 mEq/L |
| Serum HCO₃⁻ | 10 mEq/L |
| Serum glucose | 108 mg/dL |
| Serum urea nitrogen (BUN) | 14 mg/dL |
| Serum creatinine | 1.0 mg/dL |
| Serum calcium | 9.4 mg/dL |
| Serum osmolality (measured) | 317 mOsm/kg H₂O |
| Serum ethanol | undetectable |
| Serum salicylate | undetectable |
| Arterial pH (room air) | 7.26 |
| Arterial PCO₂ | 23 mm Hg |
| Urine microscopy | no crystals |
Common questions
What is a normal osmolar gap?
The osmolar gap is normally below about 10 mOsm/kg. A larger gap means an osmotically active substance is present that the calculation does not count.
How do you calculate the osmolar gap?
Subtract calculated osmolality from measured osmolality. In US units, calculated osmolality is 2 × Na⁺ + glucose/18 + BUN/2.8; in SI units it is 2 × Na⁺ + glucose + urea, all in mmol/L.
What is the difference between the osmolar gap and the anion gap?
The anion gap reveals unmeasured anions; the osmolar gap reveals unmeasured particles, charged or not. Methanol and ethylene glycol have no charge, so they raise the osmolar gap first, and the anion gap only once they become acids.
Can a toxic alcohol poisoning have a normal osmolar gap?
Yes. As the alcohol is converted into acid, the osmolar gap falls, so a patient who arrives late may have a wide anion gap with a normal osmolar gap.
How does fomepizole work?
Fomepizole competitively blocks alcohol dehydrogenase, the first enzyme in the breakdown of methanol and ethylene glycol, so no new toxic acid is made. It cannot remove acid already formed; hemodialysis does that.
Transcript
Read the full narration, by chapter687 words
0:00The puzzle: antifreeze, a normal anion gap, 35 mOsm/kg missing
An hour after drinking antifreeze, a man's anion gap is still normal. But his measured osmolality is 325, and his labs predict only 290. 35 mOsm in every kilogram of water are unaccounted for. Today you'll calculate the osmolar gap, and see why toxic alcohols open it early and the anion gap later.
0:23Calculated osmolality and the osmolar gap (US and SI formulas)
Osmolality counts every dissolved particle in a kilogram of water, and the lab measures it directly. But we can also estimate it from the three main particles: sodium, glucose and urea. In US units, calculated osmolality is 2 times sodium, plus glucose over 18, plus BUN over 2.8. The divisors just turn mg/dL into mmol/L. In SI units, it's simply 2 × sodium, plus glucose, plus urea, all in mmol/L. The osmolar gap is measured minus calculated, and it's normally below about 10. For our man: 280, plus 5, plus 5, is 290, so the gap is 35.
1:10Anion gap vs osmolar gap: why toxic alcohols raise only one at first
The anion gap reveals unmeasured anions. The osmolar gap reveals unmeasured particles, charged or not. Methanol and ethylene glycol are small molecules with no charge. So when they're swallowed, they pile up as extra particles, raising the osmolar gap but not the anion gap.
1:31Methanol → formic acid: the eyes
The real danger isn't the alcohol itself. It's what the body makes from it. For both, the first step is the enzyme alcohol dehydrogenase. Methanol becomes formaldehyde, then formic acid. Formic acid drives the acidosis and injures the retina and optic nerve, so vision blurs, can look like a snowfield, and blindness can follow.
1:56Ethylene glycol → glycolic and oxalic acid: the kidneys
Ethylene glycol becomes glycolaldehyde, then glycolic acid, and further down the line, oxalic acid. Glycolic and oxalic acid drive its acidosis. And oxalate binds calcium, so you can see calcium oxalate crystals in the urine, low calcium, and kidney injury.
2:15The twist: the osmolar gap falls as the anion gap rises
Now the twist. As the alcohol is converted into acid, the osmolar gap falls and the anion gap rises. So early on, you see a big osmolar gap with a normal anion gap. Late, the anion gap is wide, and the osmolar gap may already be back to normal. That's why a normal osmolar gap never rules out a toxic alcohol, especially in a patient who arrives late.
2:42Fomepizole, ethanol and hemodialysis
It's also why the antidote works upstream. Fomepizole competitively blocks alcohol dehydrogenase, so no new acid is made. Ethanol does the same job by competing for the enzyme. But that can't clear what's already there, so severe acidosis or organ damage needs hemodialysis, which removes the alcohol and its acids.
3:05Trap: isopropyl alcohol
One trap: isopropyl alcohol also raises the osmolar gap. But it becomes acetone, a ketone, not an acid, so you get ketosis without an anion gap acidosis.
3:18Practice question
Your turn. A 38-year-old man drank home-distilled spirits. Eighteen hours later, his vision is blurred and his optic discs look hyperemic. Sodium 138, chloride 100, bicarbonate 10, glucose 108, and BUN 14. Measured osmolality is 317. Ethanol and salicylate are undetectable, creatinine and calcium are normal, and there are no urine crystals. What's the most likely cause? A, ethanol. B, ethylene glycol. C, isopropyl alcohol. D, methanol. E, salicylate. Pause and work it out. The anion gap is 28: high. Calculated osmolality is 287, so the osmolar gap is 30. A is out: ethanol is undetectable, and it's never the sole cause of an acidosis this severe. C is out: isopropyl alcohol doesn't make an anion gap. E is out: salicylate is undetectable, and a high anion gap with an osmolar gap above 10 points to methanol or ethylene glycol. B is tempting, but there are no oxalate crystals, the kidneys and calcium are normal, and the eye findings point elsewhere. The answer is D, methanol. Both gaps are open, and formic acid explains the blurred vision and the hyperemic discs.
4:54High-yield recap and next lesson
Three things to keep. One: the osmolar gap is measured minus calculated osmolality, normally under 10. Two: starting with alcohol dehydrogenase, methanol becomes formic acid, which hits the eyes, and ethylene glycol becomes glycolic and oxalic acid, which hit the kidneys. Three: early the osmolar gap is high, late the anion gap is, and fomepizole blocks the enzyme. Next: normal anion gap acidosis, and how the urine anion gap finds its cause.
Sources
Numbers in the text, e.g. [1], point here- Lewis JL III. Metabolic Acidosis; Acid-Base Disorders. MSD Manual Professional Version (2025). msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-acidosis
- MSD Manual Professional. General Principles of Poisoning (antidote table). msdmanuals.com/professional/injuries-poisoning/poisoning/general-principles-of-poisoning
- Choy KW, Wijeratne N, Lu ZX, Doery JCG. Harmonisation of osmolal gap – can we use a common formula? Clin Biochem Rev 2016;37(3):113–119. pmc.ncbi.nlm.nih.gov/articles/PMC5111243
- Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol 2008;3(1):208–225.
- Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders. Clin J Am Soc Nephrol 2013. pmc.ncbi.nlm.nih.gov/articles/PMC3817910/
- Fomepizole injection, US prescribing information (openFDA label, effective 26 Oct 2023). api.fda.gov/drug/label.json?search=openfda.generic_name:fomepizole&limit=1
- Farkas J. Ethylene glycol & methanol poisoning. Internet Book of Critical Care (EMCrit, Jun 2024). emcrit.org/?p=437876
- Nickson C. Osmolar gap; Methanol toxicity; Ethylene glycol toxicity. LITFL. litfl.com/osmolar-gap/ · litfl.com/methanol-toxicity/ · litfl.com/ethylene-glycol-toxicity/
- WikEM. Toxic alcohols; Isopropyl alcohol toxicity. wikem.org/wiki/Toxic_alcohols · wikem.org/wiki/Isopropyl_alcohol_toxicity
- AMBOSS. Osmolar gap. amboss.com/us/snippet/Osmolar_gap
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized): bottle, liver, kidney. Eye cross-section: eye-crosssection icon by Servier (via bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/); adapted (rasterised, recoloured, outlined). All other diagrams are original.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-007.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01ABG interpretation, made simple5:35
- EP 02Anion gap explained: high vs normal gap metabolic acidosis5:25
- EP 03Winters' formula: the expected PaCO₂ in metabolic acidosis5:10
- EP 04Delta ratio: find the hidden second acid–base disorder5:09
- EP 05New bicarbonate: how the kidney excretes acid with ammonium5:23
- EP 06High anion gap metabolic acidosis causes, sorted by the anion5:54
- EP 07Osmolar gap: how toxic alcohols hide from the anion gap5:31
- EP 08Urine anion gap: telling diarrhea from RTA5:33
- EP 09Renal tubular acidosis: tell RTA types 1, 2 and 4 apart5:24
- EP 10Metabolic alkalosis: generation, maintenance and urine chloride5:30
- EP 11Vomiting metabolic alkalosis: why the urine turns acid5:36
- EP 12Bartter vs Gitelman: NKCC2, NCC and urine calcium5:21
- EP 13Respiratory acidosis compensation: acute vs chronic5:22
- EP 14Respiratory alkalosis compensation: acute vs chronic5:33
- EP 15Metabolic alkalosis compensation: the expected PaCO₂5:12
- EP 16Salicylate poisoning acid-base: a classic mixed disorder5:31
- EP 17Type A vs type B lactic acidosis: split it by mechanism5:33
- EP 18DKA anion gap explained: from high gap to hyperchloraemic acidosis5:42
- EP 19Acidosis and hyperkalemia: how pH shifts potassium5:43
- EP 20ABG interpretation in 6 steps: the acid-base algorithm5:32
- EP 21Acid-base practice cases: three ABG vignettes, one algorithm5:44