Anion Gap Calculator

Calculate anion gap and osmol gap for metabolic acidosis evaluation.

Sodium Na+ (mEq/L) (mEq/L)

Chloride Cl- (mEq/L) (mEq/L)

Bicarbonate HCO3- (mEq/L) (mEq/L)

Albumin (g/dL) for correction (g/dL)

Serum osmolality (mOsm/kg) (mOsm/kg)

0 = skip osmol gap

Glucose (mg/dL) and BUN (mg/dL) (mg/dL)

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Why Albumin Correction Is Not Optional

Albumin is the dominant unmeasured anion in plasma. Because it is negatively charged, hypoalbuminaemia lowers the anion gap by roughly 2.5 mEq/L for every 1 g/dL fall — which can mask a genuine high-gap acidosis entirely.

AG = Na⁺ − (Cl⁻ + HCO₃⁻) Corrected AG = AG + 2.5 × (4.0 − albumin)

This matters constantly in critical care, where albumin of 2 g/dL is common. An apparently normal gap of 11 in that patient corrects to 16 — genuinely raised. Skipping the correction is one of the commonest reasons a lactic acidosis or ketoacidosis is missed.

Causes by Gap Type

High anion gap (GOLDMARK)	Normal gap (hyperchloraemic)
Glycols, Oxoproline	Diarrhoea (bicarbonate loss)
L-lactate, D-lactate	Renal tubular acidosis
Methanol	Saline resuscitation
Aspirin, Renal failure	Carbonic anhydrase inhibitors
Ketoacidosis	Ureteral diversion

GOLDMARK has largely replaced the older MUDPILES mnemonic because it reflects current epidemiology — pyroglutamic acidosis from chronic paracetamol use and D-lactate from short bowel syndrome are recognised causes that MUDPILES omits.

The Delta Ratio Finds Hidden Disorders

Delta ratio = (AG − 12) / (24 − HCO₃⁻)

Delta ratio	Interpretation
< 0.4	Pure normal gap acidosis
0.4–0.8	Mixed high and normal gap
1–2	Pure high gap acidosis
> 2	Concurrent metabolic alkalosis or pre-existing high HCO₃

The logic is that in a pure high-gap acidosis, each unit of added acid consumes one bicarbonate and adds one to the gap, so the ratio is about 1. A ratio well above 2 means bicarbonate has not fallen as much as the gap rose — something else is raising it, typically vomiting or diuretics.

Worked Examples

Example 1: DKA: Na=140, Cl=99, HCO3=10, Alb=4

AG=140-99-10=31

Result: High AG=31 mEq/L — ketoacidosis

MUDPILES: D = diabetic ketoacidosis

Example 2: Normal AG acidosis: AG=10, Alb=4

Corrected AG=10 — normal gap

Result: Diarrhea, RTA, hyperchloremic acidosis

No added acid, just HCO3 loss

Example 3: Hypoalbuminaemia masking acidosis

AG 11, albumin 2.0 g/dL

Result: Corrected AG = 16 — genuinely raised

Adding 2.5 × (4.0 − 2.0) = 5. An apparently normal gap becomes clearly abnormal, which is why critical care patients always need the correction.

Example 4: Delta ratio revealing a second disorder

AG 30, HCO₃ 18

Result: Ratio = 3.0 — concurrent alkalosis

The gap rose 18 but bicarbonate fell only 6. Something is raising bicarbonate at the same time — classically vomiting alongside a ketoacidosis.

Common Mistakes

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Skipping the albumin correction

Albumin is the main unmeasured anion. At 2 g/dL the gap reads about 5 mEq/L low, which can hide a significant acidosis entirely.

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Stopping once the gap is calculated

The delta ratio reveals concurrent disorders. A high gap acidosis with a ratio above 2 has a metabolic alkalosis underneath it.

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Using a fixed normal range across laboratories

Normal gap depends on the chloride assay. Modern ion-selective electrodes give a lower reference range, typically 6–12 rather than 8–16.

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Forgetting to check the osmolal gap alongside

A high anion gap with a high osmolal gap points strongly to toxic alcohol ingestion. Checking one without the other can miss it.

Frequently Asked Questions

Winter's formula for compensation?

Expected pCO2 = 1.5×HCO3 + 8 ± 2. If actual pCO2 < expected: additional respiratory alkalosis. If > expected: additional respiratory acidosis. Delta-delta ratio: (AG-12)/(24-HCO3) for mixed disorders.

Albumin correction importance?

Each 1 g/dL decrease in albumin reduces AG by 2.5. Critically ill patients often hypoalbuminemic: AG may be 'normal' but actually elevated. Corrected AG unmasks high-AG acidosis. Always correct for albumin in ICU patients.

Why correct the anion gap for albumin?

Albumin is the principal unmeasured anion. Each 1 g/dL below 4.0 lowers the gap by about 2.5 mEq/L, potentially masking a real acidosis.

What does the delta ratio tell you?

Whether a second acid-base disorder is present. Around 1 indicates a pure high gap acidosis; above 2 suggests a concurrent metabolic alkalosis.

What is GOLDMARK?

A modern mnemonic for high anion gap acidosis: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.

Why has the normal range changed?

Modern ion-selective electrodes measure chloride differently from older methods, giving a lower reference range of roughly 6–12 mEq/L.