DLCO & KCO: Understanding Transfer Factor for MRCP Part 1

TL;DR

DLCO and KCO assess gas transfer across the alveolar–capillary membrane and are repeatedly tested in MRCP Part 1. DLCO reflects total transfer capacity, while KCO (DLCO/VA) adjusts for alveolar volume and helps localise pathology. Correct interpretation hinges on recognising characteristic patterns in emphysema, interstitial lung disease, pulmonary vascular disease, anaemia, and extrinsic restriction.

Why this topic matters for MRCP Part 1

Transfer factor questions are rarely about memorised definitions. Instead, the exam tests pattern recognition: how DLCO and KCO behave relative to spirometry and lung volumes. A single line in the stem (“DLCO reduced, KCO normal”) is often the key discriminator between parenchymal, vascular, and extrapulmonary causes of breathlessness.

This article supports the respiratory hub in the MRCP Part 1 overview and is designed to be used alongside question practice from the Crack Medicine Qbank.

Core definitions (exam-safe)

• DLCO (Diffusing capacity of the lung for carbon monoxide):Measures the total uptake of carbon monoxide from alveoli into pulmonary capillary blood. It depends on alveolar surface area, membrane thickness, and pulmonary capillary blood volume.

• KCO (Transfer coefficient):DLCO divided by alveolar volume (VA). It reflects the efficiency of gas transfer per unit lung volume.

In MRCP Part 1, KCO is not a mathematical footnote—it carries diagnostic weight.

High-yield principles you must know

1. DLCO falls when alveolar surface area is lost, the membrane is thickened, or capillary blood volume is reduced.

2. KCO helps distinguish “small lungs” from “damaged lungs.”

3. Anaemia lowers DLCO but KCO is usually normal or raised after correction.

4. Emphysema causes a reduction in both DLCO and KCO.

5. Pulmonary vascular disease causes a low DLCO with relatively preserved lung volumes.

6. In extrinsic restriction, DLCO falls but KCO is normal or increased.

7. Pulmonary haemorrhage increases DLCO due to intra-alveolar haemoglobin binding CO.

8. Smoking shortly before testing can falsely lower DLCO (carboxyhaemoglobin effect).

The 5 most tested subtopics

1. Emphysema

• Pattern: ↓ DLCO, ↓ KCO

• Mechanism: Destruction of alveolar walls reduces surface area.

• Exam tip: Helps distinguish emphysema from chronic bronchitis, where DLCO is often normal.

2. Interstitial lung disease (ILD)

• Pattern: ↓ DLCO, ↓ KCO, restrictive spirometry

• Mechanism: Thickened alveolar–capillary membrane impairs diffusion.

• Exam tip: DLCO may be the earliest abnormality in early ILD.

3. Pulmonary vascular disease

• Pattern: ↓ DLCO, normal lung volumes, KCO normal or mildly reduced

• Mechanism: Reduced pulmonary capillary blood volume.

• Exam tip: Normal spirometry with low DLCO should prompt consideration of pulmonary hypertension or chronic thromboembolic disease.

4. Anaemia

• Pattern: ↓ DLCO, KCO normal or increased after correction

• Mechanism: Less haemoglobin available to bind carbon monoxide.

• Exam tip: Always consider haemoglobin concentration when interpreting DLCO.

5. Extrinsic restrictive disorders

Examples include kyphoscoliosis, obesity, pleural disease, and neuromuscular weakness.

• Pattern: ↓ DLCO, ↑ or normal KCO

• Mechanism: Reduced lung volume with preserved alveolar units.

• Exam tip: High KCO argues against intrinsic parenchymal disease.

DLCO vs KCO: quick comparison table

Condition	DLCO	KCO	Key mechanism
Emphysema	↓	↓	Loss of alveolar surface area
ILD	↓	↓	Thickened diffusion barrier
Pulmonary vascular disease	↓	Normal/↓	Reduced capillary blood volume
Anaemia	↓	Normal/↑	Reduced haemoglobin
Extrinsic restriction	↓	↑	Preserved alveoli, low VA

Mini-case (MRCP Part 1 style)

Case: A 62-year-old man presents with progressive exertional dyspnoea. Spirometry and lung volumes are normal. DLCO is reduced, but KCO is within the normal range. Echocardiography shows right ventricular hypertrophy.

Question: What is the most likely underlying pathology?

Answer: Pulmonary vascular disease.

Explanation: Normal lung volumes exclude restriction. Reduced DLCO with preserved KCO suggests impaired pulmonary capillary blood volume rather than alveolar destruction or membrane thickening.

The 5 common exam traps

• Assuming DLCO and KCO always change in the same direction

• Forgetting to account for anaemia

• Misclassifying extrinsic restriction as interstitial lung disease

• Ignoring recent smoking before testing

• Interpreting DLCO without spirometry and lung volumes

Practical study checklist

• Learn patterns, not isolated numbers

• Always interpret DLCO alongside KCO and lung volumes

• Ask: Is this surface area, membrane, blood volume, or lung size?

• Practise mixed respiratory interpretation sets in the Crack Medicine Qbank

• Consolidate physiology using structured teaching from Crack Medicine Lectures

Frequently asked questions

What is the difference between DLCO and KCO?

DLCO measures total gas transfer, while KCO reflects transfer efficiency per unit lung volume. KCO is especially helpful when lung volumes are reduced.

Why is DLCO reduced in anaemia?

Carbon monoxide binds to haemoglobin. Reduced haemoglobin concentration lowers CO uptake despite normal alveolar structure.

Can DLCO ever be increased?

Yes. Pulmonary haemorrhage and polycythaemia can increase DLCO due to increased haemoglobin available for binding.

Is a normal DLCO enough to exclude lung disease?

No. Early interstitial lung disease or pulmonary vascular disease may still be present with near-normal values.

Ready to start?

Consolidate your understanding of DLCO and KCO by applying these patterns to real exam-style questions. Practise now with the Crack Medicine MRCP Part 1 Qbank, which includes high-yield respiratory interpretation MCQs with detailed explanations:👉 https://crackmedicine.com/qbank/

For a structured revision pathway, return to the main MRCP Part 1 hub and integrate this topic with related respiratory content and mock exams:👉 https://crackmedicine.com/mrcp-part-1/

Sources

• MRCP(UK) Examination Blueprint – https://www.mrcpuk.org/mrcpuk-examinations/mrcpuk-part-1

• ATS/ERS Standards for the Measurement of Lung Diffusing Capacity – https://www.thoracic.org/statements/resources/pft/DLCO.pdf

• British Thoracic Society guidance on pulmonary function testing – https://www.brit-thoracic.org.uk/quality-improvement/guidelines/pulmonary-function-testing/