What is a normal anion gap?

With modern analysers the normal anion gap, calculated as sodium minus (chloride plus bicarbonate), is about 8 to 12 mmol/L, although each laboratory has its own range. If potassium is added to the formula the normal range is about 4 mmol/L higher. A low albumin lowers the gap, so correct it for albumin before deciding it is normal.

What causes a high anion gap metabolic acidosis?

A high anion gap means an unmeasured acid has been added: lactic acidosis, ketoacidosis (diabetic, alcoholic or starvation), renal failure, and toxins such as methanol, ethylene glycol, salicylate and paracetamol (5-oxoproline). The GOLDMARK mnemonic lists them: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure and ketoacidosis.

What causes a normal anion gap metabolic acidosis?

A normal anion gap (hyperchloraemic) acidosis means bicarbonate has been lost or chloride gained: diarrhoea, high-output stomas and fistulas, ureteric diversions, renal tubular acidosis, acetazolamide and large volumes of 0.9% saline. The urine anion gap helps separate gut losses (negative) from renal tubular acidosis (positive).

Anion gap calculator: albumin-corrected, delta ratio

Creator: Dr. Mohammed Maree
Published: 2026-11-05
License: https://creativecommons.org/licenses/by-nc/4.0/

Written and clinically reviewed by Dr. Mohammed Maree, consultant minimally invasive and bariatric surgeon, Al-Makassed Hospital, Jerusalem, and Assistant Professor of Surgery, Al-Quds University. Last updated 5 November 2026.

Enter the results (mmol/L)

Sodium

Chloride

Bicarbonate

Potassium (optional)

Albumin (optional)

Albumin unit

Urine anion gap (optional, for a normal-gap acidosis)

Urine sodium

Urine potassium

Urine chloride

The formulas

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) normal about 8–12 mmol/L

Measure	Formula	What it tells you
Albumin-corrected gap	AG + 0.25 × (40 − albumin g/L)	Albumin is the main unmeasured anion; a low albumin hides a high gap.
Delta ratio	(AG − 12) ÷ (24 − HCO₃)	In a high-gap acidosis: is there a second disorder?
Urine anion gap	(U_Na + U_K) − U_Cl	In a normal-gap acidosis: gut (negative) or kidney (positive)?

High versus normal anion gap metabolic acidosis

High anion gap (acid added)	Normal anion gap (bicarbonate lost)
Lactic acidosis: shock, sepsis, ischaemic bowel, metformin	Diarrhoea, high-output ileostomy, pancreatic or small-bowel fistula
Ketoacidosis: diabetic, alcoholic, starvation	Ureteric diversion into bowel (ileal conduit, ureterosigmoidostomy)
Renal failure (uraemia)	Renal tubular acidosis
Toxins: methanol, ethylene glycol, salicylate, paracetamol (5-oxoproline)	Large volumes of 0.9% saline; acetazolamide

A surgical example

Day 2 after an anterior resection, the patient is drowsy with a distended abdomen. Na 138, Cl 100, HCO₃ 12, albumin 25 g/L. Anion gap = 138 − 112 = 26; corrected for albumin = 26 + 0.25 × 15 = 29.8. Delta ratio = (29.8 − 12) ÷ (24 − 12) = 1.5: a pure high anion gap acidosis. With this story, lactic acidosis from an anastomotic leak or ischaemic bowel comes first. Check the lactate and image the abdomen.

Why surgeons meet both kinds

The septic, ischaemic or shocked patient has a high-gap lactic acidosis. The patient with a high-output stoma or fistula, or who has received litres of 0.9% saline, has a normal-gap hyperchloraemic acidosis. Both can occur together, and the delta ratio is how you spot it.

References

Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007;2:162–74.
Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26:1807–10.
Mehta AN, Emmett JB, Emmett M. GOLD MARK: an anion gap mnemonic for the 21st century. Lancet 2008;372:892.
Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med 2014;371:1434–45.

Educational use

This calculator is a teaching aid. It does not replace clinical judgement, senior advice or your local guidelines.

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Anion gap calculator