Anion Gap Calculator
Calculate the serum anion gap from sodium, chloride and bicarbonate — with the albumin-corrected value that is often the more meaningful number.
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How the Anion Gap Is Calculated
The anion gap is an estimate of the unmeasured anions in serum. It is not a measured quantity — it is arithmetic:
- Anion gap = Na+ − (Cl− + HCO3−)
- Albumin-corrected gap = anion gap + 2.5 × (4.0 − albumin in g/dL)
Some laboratories include potassium: (Na+ + K+) − (Cl− + HCO3−). That version runs roughly 4 mEq/L higher, with a normal range of about 12 to 16. This calculator uses the sodium-only version, whose usual reference range is 8 to 12 mEq/L, and always reports the albumin-corrected figure alongside it.
What the Result Means
High anion gap (>12): unmeasured acid has accumulated. The classic lists are MUDPILES (methanol, uraemia, diabetic ketoacidosis, propylene glycol or paraldehyde, iron or isoniazid, lactic acidosis, ethylene glycol, salicylates) and GOLD MARK (glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketones). In practice, lactate and ketones account for most cases.
Normal anion gap (8–12): a metabolic acidosis can still be present. Hyperchloraemic acidosis — profuse diarrhoea, renal tubular acidosis, large-volume normal saline, acetazolamide, ureteric diversion — raises chloride rather than unmeasured anions, so the gap stays normal.
Low anion gap (<8): usually hypoalbuminaemia, because albumin is the largest contributor to the unmeasured anions. Laboratory interference from lithium or bromide, and paraproteinaemia, are the other recognised causes.
Why Albumin Correction Matters
Every 1 g/dL fall in serum albumin lowers the anion gap by roughly 2.5 mEq/L. In a patient with an albumin of 2.0 g/dL, an apparently normal gap of 10 is really closer to 15 — an occult high-gap acidosis. This is why the corrected value is the more reliable one in anyone who is malnourished, critically ill, nephrotic, or has chronic liver disease, and why it is reported here by default whenever an albumin value is entered.
Limitations
- Reference ranges differ between laboratories and analysers; treat 8–12 as a guide, not a universal cut-off.
- The gap identifies a pattern, not a diagnosis. Lactate, ketones, renal function, toxicology and the clinical picture are what determine the cause.
- Severe hypoalbuminaemia can mask a high-gap acidosis; severe hyperalbuminaemia is rare but shifts the gap upward.
- The delta ratio (change in gap ÷ change in bicarbonate) can help separate a pure high-gap acidosis from a mixed disorder, but it needs a reliable baseline bicarbonate.
Frequently Asked Questions
What is a normal anion gap?
Most laboratories quote 8 to 12 mEq/L for the sodium-only formula, or about 12 to 16 mEq/L when potassium is included. Always check the range printed on the report you are working from.
Why correct the anion gap for albumin?
Albumin makes up most of the unmeasured anions. When albumin falls, the gap falls with it, so a truly raised gap can look normal. Adding 2.5 mEq/L for every 1 g/dL that albumin sits below 4.0 unmasks it.
Can the anion gap be normal in a metabolic acidosis?
Yes. Hyperchloraemic acidosis — diarrhoea, renal tubular acidosis, excess saline — lowers bicarbonate while chloride rises, so the arithmetic gap is unchanged. Bicarbonate and the clinical picture are what reveal it.