Anion gap calculator

Anion gap from sodium, chloride and bicarbonate, with causes of a high gap.

Sodium (mmol/L)Chloride (mmol/L)Bicarbonate (mmol/L)

What the anion gap is for

When a blood gas or electrolytes show a metabolic acidosis (low bicarbonate), the anion gap splits the causes into two groups: acids that add an unmeasured anion (high gap) and loss of bicarbonate or gain of chloride (normal gap). It is the quickest way to narrow the differential at the bedside.

The formula

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). The normal range is about 8–12 mmol/L without potassium, but it varies between analysers, so use your lab's range. Some labs add potassium, giving a range of about 10–16.

Interpreting the result

• High anion gap acidosis: ketoacidosis (DKA, alcohol, starvation), lactic acidosis (shock, sepsis, hypoxia, metformin, aluminium phosphide poisoning), kidney failure, and toxins such as methanol, ethylene glycol and salicylates.
• Normal anion gap (hyperchloraemic) acidosis: diarrhoea and other bowel losses, renal tubular acidosis, acetazolamide, ureteric diversion, and large volumes of 0.9% saline.

Correct for albumin

Albumin is the main unmeasured anion. For every 1 g/dL that albumin falls below 4 g/dL, the expected anion gap falls by about 2.5 mmol/L. A gap of 12 in a patient with albumin 2 g/dL is really raised.

Worked example

A young man with vomiting and abdominal pain: Na 135, Cl 98, HCO₃ 10. Anion gap = 135 − 108 = 27, a high anion gap acidosis. Check capillary glucose and ketones: this is DKA until proven otherwise.

Pitfalls

• Use values from samples taken at the same time. Bicarbonate on a blood gas is calculated; the lab value is measured total CO₂. Both are fine if you are consistent.
• A normal gap does not exclude a mixed disorder. If the fall in bicarbonate is much larger or smaller than the rise in the gap, think of a second process.
• In wheat pill (aluminium phosphide) poisoning, a worsening acidosis is a sign of severity.
• Always pair the gap with the history, the pH and the lactate.

See arterial blood gas, DKA and HHS, overdose and poisoning, wheat pill poisoning, sodium bicarbonate and the textbook chapter on acid-base balance.

Frequently asked questions

A calculator supports, but does not replace, clinical judgement. Check results before acting on them.