The anion gap is Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mEq/L (12–16 if potassium is added). A gap above the normal range in metabolic acidosis points to unmeasured acids such as lactate or ketones. Correct for low albumin before calling a gap normal, and use the delta ratio to look for a second disorder.
This calculator follows StatPearls' formulas and is written for clinicians and students. It returns the gap with and without potassium, the albumin-corrected gap and the delta ratio.
How to use this calculator
- Enter sodium, chloride and bicarbonate from the same chemistry panel, in mEq/L.
- Add potassium only if your laboratory's reference range includes it.
- Add serum albumin in g/dL to get the corrected gap. This matters most in sick, malnourished or cirrhotic patients.
- Read the delta ratio, which the calculator only shows when bicarbonate is below 24.
- Compare the result with your laboratory's range and, where available, the patient's own baseline gap.
How the anion gap is calculated
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻), or (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻).
- Albumin-corrected gap = gap + 2.5 × (4.0 − albumin g/dL).
- Delta ratio = (corrected gap − 12) / (24 − HCO₃⁻). StatPearls uses 12 as the upper limit of a normal gap and 24 mEq/L as normal bicarbonate.
Blood is electrically neutral, so the "gap" is really the difference between unmeasured anions and unmeasured cations. StatPearls identifies albumin as the main unmeasured anion in health. When an acid such as lactic acid accumulates, its anion replaces bicarbonate and the gap widens. When chloride replaces the lost bicarbonate instead, the gap stays normal.
Worked examples
Each row is computed by the calculator's own functions.
| Na / Cl / HCO₃ | Albumin | Gap | Corrected | Delta ratio | Reading |
|---|---|---|---|---|---|
| 140 / 100 / 10 | 4 g/dL | 30 | 30 | 1.29 | consistent with a pure high anion gap acidosis |
| 137 / 105 / 12 | 2 g/dL | 20 | 25 | 1.08 | consistent with a pure high anion gap acidosis |
| 138 / 110 / 10 | 4 g/dL | 18 | 18 | 0.43 | suggests a concurrent normal anion gap acidosis |
| 140 / 95 / 20 | 4 g/dL | 25 | 25 | 3.25 | suggests a concurrent metabolic alkalosis |
- Row 2 shows a hidden gap. The raw gap of 20 looks only mildly raised, but with albumin 2.0 g/dL it corrects to 25. StatPearls gives a similar example: a measured gap of 10 with albumin 2.0 corrects to 15, a true high gap acidosis that might otherwise be missed.
- Row 3 has a gap of 18 but bicarbonate has fallen much further than the gap has risen, so the delta ratio of 0.43 points to an added normal gap (hyperchloremic) acidosis, as with diarrhea on top of ketoacidosis.
- Row 4 has a gap of 25 with bicarbonate only slightly low, giving a delta ratio of 3.25: a concurrent metabolic alkalosis or a raised baseline bicarbonate, as with vomiting.
Normal anion gap ranges differ
No single number is universal. StatPearls' metabolic acidosis chapter uses 8–12 mEq/L (12–16 with potassium), its biochemistry chapter quotes 4–12 mmol/L depending on phosphate and albumin, and its DKA chapter treats a gap above 14–15 mEq/L as raised. The 2024 ADA-led consensus report uses above 12 mmol/L for DKA. These differences partly reflect analyzers, so always check the reference range printed on your laboratory's report.
High anion gap causes (GOLD MARK)
| Letter | Cause | Unmeasured anion |
|---|---|---|
| G | Glycols (ethylene, propylene) | Glycolate, oxalate, lactate |
| O | Oxoproline (chronic acetaminophen) | 5-oxoproline |
| L | L-lactate (shock, sepsis, metformin) | Lactate |
| D | D-lactate (short bowel syndrome) | D-lactate |
| M | Methanol | Formate |
| A | Aspirin (salicylates) | Lactate, ketoacids |
| R | Renal failure, rhabdomyolysis | Sulfate, phosphate |
| K | Ketones (diabetic, alcoholic, starvation) | β-hydroxybutyrate, acetoacetate |
| T | Toluene | Hippurate |
When toxic alcohols are possible, StatPearls pairs the anion gap with the osmolar gap: measured minus calculated osmolality, where calculated osmolality = 2 × (Na + K) + glucose/18 + BUN/2.8 + ethanol/4.6. A gap above 10 mOsm/kg with a high anion gap strongly suggests toxic alcohol ingestion, though the osmolar gap can be normal early on.
Normal anion gap acidosis
StatPearls names diarrhea and renal tubular acidosis as the main causes of normal gap (hyperchloremic) acidosis. Large volumes of normal saline are another common cause: it carries 154 mEq/L of chloride against roughly 100–106 mEq/L in plasma and contains no bicarbonate. The older mnemonic USED CARP covers ureterostomy, small bowel fistula, extra chloride, diarrhea, carbonic anhydrase inhibitors, adrenal insufficiency, renal tubular acidosis and pancreatic fistula.
To separate gut from kidney losses, StatPearls uses the urine anion gap (urine Na + K − Cl). A negative value suggests appropriate ammonium excretion and a gastrointestinal cause; a positive value suggests reduced ammonium excretion, as in distal renal tubular acidosis or chronic kidney disease.
Common mistakes
- Skipping the albumin correction. Low albumin lowers the gap and can mask a high gap acidosis, which StatPearls flags as a particular issue in ICU patients.
- Mixing formulas and ranges. If you add potassium, compare against the with-potassium range (12–16), not 8–12.
- Ignoring a low gap. An unexpectedly low value can point to a paraprotein, lithium toxicity or a measurement error.
- Reading the delta ratio without a raised gap. It only has meaning once a high anion gap acidosis is established.
- Stopping at the gap. Check that respiratory compensation is appropriate with the Winter's formula calculator, and in hyperglycemia use the corrected sodium calculator.
For healthcare professionals and students only. An anion gap is one clue, not a diagnosis, and this tool offers no medical advice. Read it against the patient's baseline gap, the history and your laboratory's own range. Suspected toxic ingestion or DKA needs urgent clinical care. Patients should ask their own doctor to explain their results.
Frequently asked questions
How do you calculate the anion gap?
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). Some labs include potassium: (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻). StatPearls notes potassium is often left out because its contribution is small.
What is a normal anion gap?
StatPearls gives 8–12 mEq/L without potassium and 12–16 mEq/L with it. Its biochemistry chapter quotes 4–12 mmol/L, and ranges depend on the analyzer, so use your lab's reference range.
How do you correct the anion gap for albumin?
Corrected AG = AG + 2.5 × (4.0 − albumin in g/dL). For example Na 137, Cl 105, HCO₃ 12 gives a gap of 20; with albumin 2.0 g/dL the corrected gap is 25.
How do you correct the anion gap for albumin in g/L?
StatPearls gives the SI version as a fall of about 0.25 mmol/L in the anion gap for every 1 g/L fall in albumin, or 2.3 mmol/L per 10 g/L. Albumin in g/L divided by 10 gives g/dL for the formula here.
What is the delta ratio?
Delta ratio = (anion gap − 12) / (24 − HCO₃⁻). 1–2 suggests a pure high anion gap metabolic acidosis, below 1 a concurrent normal anion gap acidosis, above 2 a concurrent metabolic alkalosis. For Na 140, Cl 100, HCO₃ 10 the gap is 30 and the delta ratio 1.29.
What causes a high anion gap metabolic acidosis?
StatPearls uses the GOLD MARK(e)T mnemonic: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin (salicylates), renal failure and rhabdomyolysis, ketones, and toluene. CAT MUDPILES is an older alternative.
What causes a normal anion gap metabolic acidosis?
StatPearls names diarrhea and renal tubular acidosis as the main causes. Others include large volumes of normal saline, ureteral diversions, pancreatic fistulas, carbonic anhydrase inhibitors, Addison disease and some TPN regimens.
What causes a low anion gap?
Most often low albumin, the main unmeasured anion. StatPearls also lists paraproteins such as IgG in myeloma, lithium intoxication, severe hypertriglyceridemia, low phosphate and raised unmeasured cations such as magnesium. A low gap can also flag a lab measurement error.
Is an anion gap above 20 always serious?
StatPearls states that an anion gap above 18 almost always indicates an organic acid, such as lactate or ketones, and that kidney failure alone rarely produces a gap above 18. It needs prompt clinical assessment.
What anion gap is used for DKA?
The 2024 ADA-led consensus report states that an anion gap above 12 mmol/L indicates a high anion gap metabolic acidosis consistent with DKA, and that about one-third of people with DKA have a mixed acid-base disorder. StatPearls notes insulin is continued until the anion gap closes.
Sources & method
- StatPearls (NCBI Bookshelf) — Anion gap and non-anion gap metabolic acidosis (formula, 8–12 range, albumin correction, delta ratio, GOLD MARK, osmolar and urine gaps)
- StatPearls (NCBI Bookshelf) — Biochemistry, anion gap (4–12 mmol/L, low anion gap causes, USED CARP)
- StatPearls (NCBI Bookshelf) — Adult diabetic ketoacidosis
- Umpierrez et al. (2024), Diabetes Care — Hyperglycemic crises in adults with diabetes: a consensus report
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