Mixed Acid–Base Disorders & Compensation — MRCP Part 1

TL;DR

Mixed acid–base disorders are a high-yield, frequently tested topic in MRCP Part 1 because they assess physiology, pattern recognition, and clinical reasoning together. The key to scoring marks is recognising inappropriate compensation and understanding that a near-normal pH often hides dual pathology. This article provides exam-focused rules, common patterns, a worked mini-case, and a practical revision checklist.

Why this topic matters for MRCP Part 1

Acid–base interpretation is a core competency tested repeatedly in MRCP exams. Mixed disorders are deliberately used to differentiate candidates who apply a structured approach from those relying on isolated values or memorised formulas.

In exam questions, you are expected to:

• Identify the primary acid–base abnormality.

• Decide whether compensation is appropriate.

• Detect the presence of more than one primary disorder.

A systematic method prevents common errors and improves speed under time pressure.

For a syllabus-level overview, see the official MRCP(UK) guidance:https://www.mrcpuk.org/mrcpuk-examinations/part-1

Core principles you must apply every time

Use this fixed sequence in every question stem:

1. Assess the pH – acidaemia, alkalaemia, or near-normal.

2. Identify the primary process – metabolic (HCO₃⁻) or respiratory (PaCO₂).

3. Check compensation – is it appropriate for the primary disorder?

4. If compensation is inappropriate, a mixed disorder exists.

5. In metabolic acidosis, calculate the anion gap.

6. In high anion gap metabolic acidosis, assess the delta gap.

7. Differentiate acute vs chronic respiratory disorders using bicarbonate change.

8. Remember: compensation never overcorrects pH to the opposite side.

9. Integrate clinical context – physiology always reflects disease.

These steps align with standard teaching in core medical textbooks such as Kumar & Clark’s Clinical Medicine.

Expected compensation: what examiners expect you to know

Primary disorder	Expected compensatory change	Key exam point
Metabolic acidosis	↓ PaCO₂ ≈ 1.2 mmHg per 1 mmol/L ↓ HCO₃⁻	Use Winter’s concept
Metabolic alkalosis	↑ PaCO₂ ≈ 0.6–0.7 mmHg per 1 mmol/L ↑ HCO₃⁻	Compensation limited by hypoxia
Acute respiratory acidosis	↑ HCO₃⁻ ≈ 1 mmol/L per 10 mmHg ↑ PaCO₂	Minimal renal response
Chronic respiratory acidosis	↑ HCO₃⁻ ≈ 4 mmol/L per 10 mmHg ↑ PaCO₂	Indicates chronic disease
Acute respiratory alkalosis	↓ HCO₃⁻ ≈ 2 mmol/L per 10 mmHg ↓ PaCO₂	Seen early in sepsis
Chronic respiratory alkalosis	↓ HCO₃⁻ ≈ 4–5 mmol/L per 10 mmHg ↓ PaCO₂	Pregnancy, liver disease

Exam rule: If measured values fall outside the expected range, there is an additional primary disorder.

The 5 most tested mixed acid–base patterns

1. Sepsis – high anion gap metabolic acidosis + respiratory alkalosis.

2. COPD with vomiting or diuretics – respiratory acidosis + metabolic alkalosis.

3. Salicylate toxicity – respiratory alkalosis + high anion gap metabolic acidosis.

4. Renal failure with diarrhoea – high anion gap + non-anion gap metabolic acidosis.

5. Cardiorespiratory arrest – metabolic acidosis + acute respiratory acidosis.

These patterns appear repeatedly in question banks and past exam recalls.

Worked mini-case (MRCP style)

Stem: A 58-year-old man is admitted with pneumonia and sepsis. Arterial blood gas on air shows:

• pH 7.37

• PaCO₂ 27 mmHg

• HCO₃⁻ 15 mmol/L

Stepwise interpretation:

• pH is near normal → suspect mixed disorder.

• Low HCO₃⁻ → metabolic acidosis.

• Expected PaCO₂ ≈ 1.2 × (24 − 15) ≈ 11 mmHg fall → expected PaCO₂ ≈ 29 mmHg.

• Actual PaCO₂ is 27 mmHg → lower than expected.

Conclusion: Mixed metabolic acidosis and respiratory alkalosis, consistent with sepsis.

Why this scores marks: Normal or near-normal pH is a classic distractor in MRCP Part 1.

Common exam traps (high-yield)

• Assuming normal pH means no acid–base disorder.

• Forgetting to assess compensation.

• Calling over-compensation “physiological”.

• Ignoring delta gap analysis in high anion gap acidosis.

• Failing to match ABG patterns to clinical context.

Practical revision checklist

• Always follow the same interpretation order.

• Memorise direction and magnitude of compensation, not formulas alone.

• Practise recognising common paired disorders.

• Avoid over-calculation; MRCP answers rely on trends.

• Reinforce learning with timed MCQs and full mock exams.

For structured practice, use:

• MRCP Part 1 hub: https://www.crackmedicine.com/mrcp-part-1/

• Question bank: https://www.crackmedicine.com/qbank/

• Mock tests: https://www.crackmedicine.com/mock-tests/

FAQs

How do you quickly identify a mixed acid–base disorder?

By finding inappropriate compensation or a near-normal pH with abnormal PaCO₂ and HCO₃⁻.

Can compensation ever fully normalise pH?

No. Complete normalisation suggests a second primary disorder.

Is Winter’s formula essential for MRCP Part 1?Understanding the principle is sufficient; exact calculations are rarely required.

Why is respiratory alkalosis common in sepsis?

Inflammatory mediators stimulate hyperventilation early in sepsis.

Ready to start?

Consolidate mixed acid–base disorders with targeted practice and exam-style timing.

• Revise the full syllabus: https://www.crackmedicine.com/mrcp-part-1/

• Practise high-yield ABG MCQs: https://www.crackmedicine.com/qbank/

• Test exam readiness with timed mocks: https://www.crackmedicine.com/mock-tests/

Sources

• MRCP(UK) Examination Syllabus – https://www.mrcpuk.org

• Kumar & Clark’s Clinical Medicine.

• Life in the Fast Lane (Acid–Base Disorders) – https://litfl.com/acid-base-disorders/