What is the normal anion gap?

The normal anion gap is typically **8–12 mEq/L** using the formula *Na⁺ − (Cl⁻ + HCO₃⁻)*. Note that older clinical literature and some laboratories cite a reference range of up to *16 mEq/L* when potassium is included in the formula — always confirm your lab's reference range.

Why does hypoalbuminaemia affect the anion gap?

Albumin is a *negatively charged protein* that contributes significantly to the unmeasured anions. When albumin is low, the baseline AG falls, and a *true high-gap acidosis can be masked*. The albumin-corrected AG adds 2.5 mEq/L for every 1 g/dL that albumin falls below 4.0 g/dL.

What does MUDPILES stand for?

**MUDPILES** is a mnemonic for causes of high anion gap metabolic acidosis: *Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol/Paracetamol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.*

What is the delta-delta ratio?

The **delta-delta ratio** = (AG − 12) ÷ (24 − HCO₃). It helps identify *mixed acid-base disorders*. A ratio below 1 suggests a co-existing non-gap metabolic acidosis; between 1–2 is consistent with a pure high-gap acidosis; above 2 suggests a co-existing metabolic alkalosis.

When should I calculate the osmolal gap?

Calculate the osmolal gap whenever a **toxic alcohol ingestion** is suspected (methanol, ethylene glycol, isopropanol), particularly in patients with a high anion gap acidosis and visual disturbance, altered consciousness, or unexplained metabolic decompensation.

Can a normal anion gap rule out toxic alcohol ingestion?

**No.** In the *late stages* of methanol or ethylene glycol toxicity, the parent alcohol has been metabolised to organic acids. At this point the osmolal gap may be normal while the anion gap is markedly elevated. Clinical suspicion must guide testing regardless of the AG result.

Laboratory · Acid-Base Analysis

Anion Gap
Calculator

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Important Notice Before Use

Last reviewed: March 2026

This is an educational reference tool only. Acid-base interpretation requires integration with the full clinical picture, blood gas results, and clinical context.

The default normal AG of 12 mmol/L is a historical value. Many modern analysers report a lower reference range (6–10 mmol/L without K). Always verify your laboratory's specific reference range before interpreting results.

I understand this is educational and will verify my lab's reference ranges before use.