Anion Gap
Calculates anion gap to evaluate causes of metabolic acidosis (MUDPILES vs NAGMA).
Score interpretation
Anion gap 8–12 mmol/L: Normal. No high anion gap acidosis.
→ If metabolic acidosis with normal AG: consider non-anion gap causes (USED CARP mnemonic: Ureteral diversion, Saline, Enteric losses, Diarrhoea, Chloride, Addison's, RTA, Pancreatic fistula).
Anion gap 12–16 mmol/L: Mildly elevated. Borderline.
→ Correlate with clinical picture. Check albumin (each 10g/L below 40 reduces AG by ~2.5). Correct AG for albumin.
Anion gap > 16 mmol/L: High anion gap metabolic acidosis (HAGMA).
→ MUDPILES: Methanol, Uraemia, DKA/AKA, Propylene glycol/Paracetamol, Isoniazid/Infection, Lactic acidosis, Ethylene glycol, Salicylates.
Interpretation bands for the Anion Gap. Apply clinical judgement and local guidance.
References
- Emmett M, Narins RG. Clinical use of the anion gap. Medicine. 1977.
Related
Curated clinical cross-links plus same-class fallbacks.
- Difficult Airway Algorithm (DAS) · DAS 2015; Royal College of Anaesthetists
- Major Haemorrhage Protocol · NICE NG24; UK MHP guidelines
- New-Onset Atrial Fibrillation · ESC 2020 AF Guidelines; NICE NG196
- Hypertensive Emergency · ESC/ESH 2018 Hypertension Guidelines; NICE NG136
- Bradycardia Management · Resuscitation Council UK ABCDE; ESC 2021 Pacing Guidelines
- Ventricular Tachycardia / Fibrillation · Resuscitation Council UK ACLS; ESC 2022 Ventricular Arrhythmia Guidelines
Featured in these MRCEM clinical pathways
The Anion Gap is covered in detail — with RCEM/NICE evidence base, indications and pitfalls — in the following exam-focused pathways on our sister siteReviseMRCEM.
MRCEM Primary / Intermediate / OSCE candidates: each pathway includes exam-style questions, RCEM/NICE citations, and FAQ summaries.
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.
