ABG Masterclass: The 6-Step Algorithm for MRCP Part 1

TL;DR

ABG interpretation is a reliable scoring area if approached systematically. This article outlines a clinician-safe 6-step algorithm for MRCP Part 1, highlights the most tested patterns and traps, and provides an exam-style MCQ with explanation. Use it alongside regular question practice to eliminate avoidable acid–base errors.

Why ABGs Matter in MRCP Part 1

Arterial blood gas (ABG) questions appear frequently in MRCP Part 1 because they integrate respiratory physiology, renal compensation, and acute medicine decision-making. The exam rewards candidates who follow a structured method rather than pattern recognition alone. Most lost marks come from missed mixed disorders, misjudged compensation, or ignored oxygenation data.

The official MRCP(UK) blueprint confirms that interpretation of investigations, including ABGs, is a core competency assessed across multiple specialties (MRCP(UK): https://www.mrcpuk.org/mrcpuk-examinations/mrcpuk-part-1).

The 6-Step ABG Algorithm (Exam-Safe)

Step 1 — Confirm validity and context

• Arterial (not venous) sample

• Inspired oxygen (room air vs supplemental O₂)

• Clinical context: COPD, sepsis, vomiting, renal failure

MRCP stems often embed the diagnosis in the history—use it.

Step 2 — Assess the pH

• pH < 7.35 → acidaemia

• pH > 7.45 → alkalaemia

• A near-normal pH does not exclude pathology (think mixed disorders).

Step 3 — Identify the primary disturbance

Match pH with PaCO₂ and HCO₃⁻:

• ↓ pH + ↑ PaCO₂ → respiratory acidosis

• ↓ pH + ↓ HCO₃⁻ → metabolic acidosis

• ↑ pH + ↓ PaCO₂ → respiratory alkalosis

• ↑ pH + ↑ HCO₃⁻ → metabolic alkalosis

Step 4 — Check compensation

Ask whether compensation is appropriate.

• Metabolic acidosis: Winter’s formulaExpected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2

• Metabolic alkalosis: PaCO₂ rises ~0.7 kPa per 10 mmol/L HCO₃⁻

• Respiratory disorders: distinguish acute vs chronic patterns.

Inappropriate compensation = mixed acid–base disorder (high-yield).

Step 5 — Calculate the anion gap (if metabolic acidosis)

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

• Normal anion gap: diarrhoea, renal tubular acidosis

• High anion gap: lactate, ketones, uraemia, toxins

Correct for albumin if provided (frequent exam trap).

Step 6 — Evaluate oxygenation

• Compare PaO₂ to FiO₂

• Consider A–a gradient if values are given

• Hypoxaemia with normal PaCO₂ suggests V/Q mismatch rather than hypoventilation

Guidance on oxygen interpretation aligns with British Thoracic Society standards:https://www.brit-thoracic.org.uk/quality-improvement/guidelines/emergency-oxygen/

High-Yield ABG Patterns (Memorise)

Pattern	pH	PaCO₂	HCO₃⁻	Classic MRCP Association
Acute respiratory acidosis	↓	↑	Normal	Opiate overdose
Chronic respiratory acidosis	↓/Normal	↑↑	↑	Stable COPD
Metabolic acidosis (high AG)	↓	↓	↓	Sepsis, DKA
Metabolic alkalosis	↑	↑	↑	Vomiting, diuretics
Respiratory alkalosis	↑	↓	Normal	Pulmonary embolism

The 5 Most Tested ABG Scenarios

1. COPD with acute decompensation (acute on chronic respiratory acidosis)

2. Diabetic ketoacidosis vs lactic acidosis

3. Vomiting and diuretic use causing metabolic alkalosis

4. Sepsis-related lactic acidosis

5. Salicylate poisoning (mixed respiratory alkalosis + metabolic acidosis)

Mini-MCQ (Exam Style)

A 70-year-old man with severe COPD presents with confusion. ABG on air:pH 7.31, PaCO₂ 9.2 kPa, HCO₃⁻ 36 mmol/L, PaO₂ 7.0 kPa.

What is the acid–base diagnosis?

A. Acute respiratory acidosisB. Chronic respiratory acidosisC. Acute on chronic respiratory acidosisD. Metabolic acidosisE. Metabolic alkalosis

Correct answer: C

Explanation: The raised PaCO₂ with elevated bicarbonate indicates chronic respiratory acidosis. Persistent acidaemia confirms acute decompensation on a chronic background—a classic MRCP pattern.

Common Pitfalls (Exam Traps)

• Skipping compensation assessment

• Assuming normal pH = normal ABG

• Forgetting albumin correction for anion gap

• Missing mixed acid–base disorders

• Ignoring oxygenation data entirely

Practical Revision Checklist

• Interpret 5 ABGs daily using the same 6 steps

• Memorise Winter’s formula and COPD compensation rules

• Practise mixed disorders using question banks

• Review incorrect answers, not just scores

• Revisit ABGs in the final 2 weeks before the exam

FAQs (People Also Ask)

Is ABG interpretation high-yield for MRCP Part 1?Yes. It integrates physiology and acute medicine and appears frequently.

Do I need to memorise formulas?

Winter’s formula and basic compensation patterns are commonly tested.

How do I recognise mixed acid–base disorders quickly?

Look for inappropriate compensation or conflicting pH and gas values.

Are ABGs tested outside respiratory medicine?

Yes—renal, endocrine, toxicology, and sepsis questions frequently use ABGs.

Ready to start?

Consolidate this algorithm by applying it to real exam-style questions in our Free MRCP MCQs, then pressure-test your timing with a mock test. For structured revision, see our Study plan for MRCP Part 1.

Sources

• MRCP(UK) Part 1 examination guidance: https://www.mrcpuk.org/mrcpuk-examinations/mrcpuk-part-1

• British Thoracic Society Emergency Oxygen Guideline:https://www.brit-thoracic.org.uk/quality-improvement/guidelines/emergency-oxygen/

• Oxford Handbook of Clinical Medicine (Acid–Base Disorders chapter)