Anion Gap vs Non-Anion Gap Acidosis — MRCP Part 1
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Anion Gap vs Non-Anion Gap Acidosis — MRCP Part 1

TL;DR;

In MRCP Part 1, metabolic acidosis questions often hinge on whether the anion gap is raised or normal. A raised anion gap indicates accumulation of unmeasured acids (e.g. ketoacidosis, lactic acidosis), while a normal (non-anion gap) acidosis reflects bicarbonate loss with chloride retention. Rapid calculation, cause recognition, and awareness of exam traps are essential for scoring marks.


Why this topic matters in MRCP Part 1

Metabolic acidosis is a recurring favourite in MRCP Part 1 because it tests core physiology, renal medicine, and acute care decision-making in a compact way. Examiners expect candidates to interpret minimal biochemical data, classify the acidosis correctly, and identify the most likely cause without overthinking.

This topic commonly appears alongside:

  • Arterial blood gas (ABG) interpretation

  • Diabetic emergencies

  • Sepsis and shock

  • Chronic kidney disease

A strong grasp of anion gap logic allows you to answer these questions quickly and safely under exam conditions.

For an overview of how this fits into the wider syllabus, see the MRCP Part 1 overview:👉 https://www.crackmedicine.com/mrcp-part-1/


What is the anion gap?

The anion gap (AG) estimates the concentration of unmeasured anions in plasma.

Formula (exam standard)

Anion gap = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)

In many MRCP questions, potassium is omitted:

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

Normal range

  • 8–12 mmol/L (without potassium)

A raised anion gap implies accumulation of acids that are not routinely measured.


Anion gap vs non-anion gap metabolic acidosis

Key comparison

Feature

Anion Gap Metabolic Acidosis (AGMA)

Non-Anion Gap Metabolic Acidosis (NAGMA)

Main mechanism

Acid accumulation

Bicarbonate loss

Bicarbonate

Low

Low

Chloride

Normal

High (hyperchloraemia)

Anion gap

Raised

Normal

Common settings

DKA, sepsis, renal failure

Diarrhoea, RTA, saline excess

Exam shortcut:

Low HCO₃⁻ + high Cl⁻ = think non-anion gap acidosis

The 5 most tested causes of high anion gap acidosis

MRCP now favours the GOLD MARK mnemonic:

  1. Glycols – ethylene glycol, propylene glycol

  2. Oxoproline – chronic paracetamol use

  3. L-lactate – sepsis, hypoxia, shock

  4. D-lactate – short bowel syndrome

  5. Methanol

  6. Aspirin (salicylates)

  7. Renal failure (uraemia)

  8. Ketoacidosis (diabetic, alcoholic, starvation)

Among these, DKA, lactic acidosis, and uraemia are the most frequently tested in MRCP Part 1.


The 5 most tested causes of non-anion gap acidosis

  1. Diarrhoea (gastrointestinal bicarbonate loss)

  2. Renal tubular acidosis (types 1, 2, and 4)

  3. Excess 0.9% saline infusion

  4. Pancreatic or biliary fistulae

  5. Ureteric diversion (e.g. ileal conduit)

These are often framed as “normal anion gap” or “hyperchloraemic” acidosis in question stems.


Medical student revising metabolic acidosis concepts for MRCP Part 1

How MRCP Part 1 typically asks this

A classic MRCP question will:

  • Provide Na⁺, Cl⁻, and HCO₃⁻ only

  • Ask for the most likely diagnosis or underlying cause

  • Include a short clinical vignette (e.g. vomiting, sepsis, diabetes)

You are rarely asked to manage the patient in detail—classification and cause are the priority.

To practise these stems in exam format, use a high-quality MRCP question bank such as:👉 https://www.crackmedicine.com/qbank/


Mini-case (exam style)

Question A 26-year-old man with type 1 diabetes presents with abdominal pain and vomiting. Blood results show:

  • Na⁺ 140 mmol/L

  • Cl⁻ 100 mmol/L

  • HCO₃⁻ 10 mmol/L

What best describes his acid–base status?

Step 1: Calculate the anion gapAG = 140 − (100 + 10) = 30 mmol/L

Step 2: Interpret

  • Anion gap is raised

  • Metabolic acidosis present

Correct answer:👉 High anion gap metabolic acidosis, most consistent with diabetic ketoacidosis


Five common MRCP traps

  1. Ignoring albumin

    • Low albumin reduces the anion gap and may mask AGMA.

  2. Calling hyperchloraemic acidosis “normal”

    • Normal anion gap ≠ normal biochemistry.

  3. Assuming all renal failure causes AGMA

    • Early CKD may cause non-anion gap acidosis.

  4. Missing mixed acid–base disorders

    • DKA with vomiting can partially normalise pH.

  5. Using outdated mnemonics only (MUDPILES)

    • GOLD MARK is now preferred in modern exams.


Practical MRCP Part 1 study checklist

  • Memorise the anion gap formula and normal range

  • Practise fast mental calculation

  • Associate high chloride with non-anion gap acidosis

  • Learn GOLD MARK causes cold

  • Revise renal tubular acidosis patterns

  • Test yourself regularly using full-length mock exams:👉 https://www.crackmedicine.com/mock-tests/


How this fits into your revision plan

This topic links directly with ABG interpretation and acute medicine. A logical next step is revising ABG interpretation for MRCP, then consolidating with timed questions.

You can also reinforce this area with structured teaching via focused MRCP lectures:👉 https://www.crackmedicine.com/lectures/


FAQs

What is the normal anion gap in MRCP exams?

Usually 8–12 mmol/L without potassium. Values outside this range should prompt cause identification.

Why does diarrhoea cause non-anion gap acidosis?

Because bicarbonate is lost from the gut, with compensatory chloride retention keeping the anion gap normal.

Is potassium included in the anion gap calculation?

It can be, but many MRCP questions omit it. Consistency matters more than inclusion.

Does hypoalbuminaemia affect the anion gap?

Yes. Low albumin reduces the anion gap and can hide high anion gap acidosis.


Ready to start?

If you are revising metabolic disorders for MRCP Part 1, do not rely on passive reading alone. Consolidate this topic by practising real exam-style questions and timed papers:


Sources

 
 
 
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