Anion Gap Calculator

Anion Gap Calculator for Nephrology. With the common K+-excluded formula, a normal gap is about 8-12 mEq/L (modern ion-selective analyzers often run 3-11); values above ~12 after albumin correction indicate a high-gap acidosis whose classic causes are lactate, ketoacids, renal failure (uremia), and toxins such as methanol, ethylene glycol, and salicylates. A normal or low gap with acidosis points to bicarbonate loss (diarrhea, renal tubular acidosis) or early kidney disease. A genuinely low or negative gap (usually from severe hypoalbuminemia, but also lithium, bromide, or cationic paraproteins as in myeloma) is itself a diagnostic clue. In an established high-gap acidosis, pairing the gap with the delta ratio helps unmask a coexisting normal-gap acidosis or metabolic alkalosis.

How this calculator works

The anion gap estimates unmeasured plasma anions from routine electrolytes: AG = Na+ − (Cl− + HCO3−). Some labs and clinicians include potassium (AG = [Na+ + K+] − [Cl− + HCO3−]), which raises the reference range by roughly 4 mEq/L. Because it is derived, the gap is only as reliable as its inputs, so hypoalbuminemia must be accounted for: the albumin-corrected AG adds 2.5 mEq/L for every 1 g/dL that measured albumin falls below 4 g/dL (0.25 mEq/L per g/L on SI scale).

When to use this calculator

Use it on every basic metabolic panel to screen for occult metabolic acidosis and to sort acidosis into high-gap versus normal-gap (hyperchloremic) categories, which drives entirely different differentials. It is most valuable in the undifferentiated sick patient (altered mental status, sepsis, toxic ingestion, unexplained tachypnea) and in monitoring DKA resolution. Do not treat the raw gap in isolation when albumin is abnormal, and do not use it as a substitute for lactate, ketones, or a toxicology workup when those are clinically indicated.

Inputs used

  • Sodium
  • Chloride
  • Bicarbonate
  • Albumin when correcting the gap

Clinical interpretation

With the common K+-excluded formula, a normal gap is about 8-12 mEq/L (modern ion-selective analyzers often run 3-11); values above ~12 after albumin correction indicate a high-gap acidosis whose classic causes are lactate, ketoacids, renal failure (uremia), and toxins such as methanol, ethylene glycol, and salicylates. A normal or low gap with acidosis points to bicarbonate loss (diarrhea, renal tubular acidosis) or early kidney disease. A genuinely low or negative gap (usually from severe hypoalbuminemia, but also lithium, bromide, or cationic paraproteins as in myeloma) is itself a diagnostic clue. In an established high-gap acidosis, pairing the gap with the delta ratio helps unmask a coexisting normal-gap acidosis or metabolic alkalosis.

Worked example

A patient has Na+ 140, Cl− 100, HCO3− 12, albumin 2.0 g/dL. Raw AG = 140 − (100 + 12) = 28 mEq/L, clearly elevated. Correcting for albumin: 2 g/dL below normal x 2.5 = +5, giving a corrected AG of 33 mEq/L, confirming a large high-gap metabolic acidosis and prompting a search for lactate, ketones, and toxic alcohols. Had albumin been 2.0 and the raw AG only 14 (falsely reassuring), the corrected value of 19 would still flag a true gap acidosis that the uncorrected number missed.

Limitations and safety notes

The reference range is analyzer- and formula-dependent (whether K+ is included, and the chloride method used), so a single fixed cutoff is unreliable across labs. Hypoalbuminemia is the dominant confounder: each 1 g/dL drop lowers the observed gap by ~2.5 mEq/L and can fully mask a lactic or ketoacidosis in critically ill patients, roughly half of whom have albumin under 2 g/dL. Severe hyperlipidemia, lithium or bromide intoxication, IgG paraproteinemia, and marked hypernatremia or hyponatremia also distort the value. A normal gap never excludes a metabolic acidosis when the abnormality is hyperchloremic.

Frequently asked questions

Should potassium be included in the calculation?

Both conventions are valid. Excluding K+ (the more common form) gives a normal range near 8-12 mEq/L; including K+ shifts it up by about 4 mEq/L. What matters is that you apply the same formula and reference range consistently and know which one your lab reports.

Why correct the anion gap for albumin?

Albumin is the largest contributor to the normal unmeasured-anion pool, so a low albumin lowers the measured gap. In hypoalbuminemic patients a 'normal' gap can hide a significant lactic or ketoacidosis. Add 2.5 mEq/L per 1 g/dL that albumin is below 4 g/dL to recover the true gap.

What causes a high anion gap?

The high-gap causes are the unmeasured acids: lactate (shock, sepsis, ischemia), ketoacids (DKA, alcoholic and starvation ketoacidosis), uremic anions in renal failure, and toxic ingestions including methanol, ethylene glycol, and salicylates. A gap above 20-25 strongly implies an organic acidosis regardless of the bicarbonate.

What does a low or negative anion gap mean?

The commonest cause is severe hypoalbuminemia. Other causes include cationic paraproteins (multiple myeloma), lithium or bromide intoxication, and severe hypercalcemia or hypermagnesemia. A negative gap should prompt a check for lab error or one of these specific conditions.

Does a normal anion gap rule out acidosis?

No. Normal-gap (hyperchloremic) metabolic acidosis from diarrhea, renal tubular acidosis, or early chronic kidney disease keeps the gap normal because bicarbonate loss is matched by a chloride rise. Always interpret the gap alongside the bicarbonate and pH, not on its own.

References

  • Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore). 1977;56(1):38-54. PMID: 401925.
  • Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807-10. PMID: 9824071.
  • Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore). 1980;59(3):161-87. PMID: 6774200.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 10, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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