Patient guide · Pulmonary function
Gas transfer test (TLCO / DLCO), measuring how efficiently oxygen crosses the alveolar-capillary membrane.
The gas transfer test (TLCO / DLCO — transfer factor for carbon monoxide) measures how efficiently the lungs move gas from alveoli into blood. Detects interstitial lung disease, pulmonary vascular disease, emphysema and anaemia effects.
Key facts
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Transfer factor for CO
Transfer factor for carbon monoxide — TLCO / DLCO — measures how efficiently gas moves from alveoli into blood.
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Single-breath 10-second manoeuvre
A calibrated inhaled test-gas mixture is held for approximately ten seconds and the uptake calculated.
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Corrected for haemoglobin
Results are adjusted for haemoglobin, since anaemia reduces measured uptake independently of the lungs.
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Complements spirometry and lung volumes
Interpreted alongside spirometry and lung volumes for a full picture of pulmonary function.
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Reduced in ILD, emphysema, PH
Falls in interstitial lung disease, emphysema and pulmonary hypertension — often the earliest signal.
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Increased in asthma, alveolar haemorrhage
Rises in acute asthma and in alveolar haemorrhage, where free blood in the alveoli binds carbon monoxide.
Preparation and the manoeuvre
From referral to reproducible result — what happens, in order.
The gas transfer test is quick, non-invasive and technique-dependent — done well, it is one of the most sensitive tests in pulmonary medicine.
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Respiratory referral
A referral from a respiratory consultant or GP with a clear clinical question.
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No smoking 4 hours before
Carboxyhaemoglobin from recent smoking artificially lowers the measured transfer factor.
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Withhold bronchodilators if requested
Some protocols ask for short- or long-acting bronchodilators to be held — the department will confirm.
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Baseline spirometry + lung volumes
Full pulmonary function including spirometry and static lung volumes is measured on the same visit.
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Single-breath TLCO manoeuvre
You breathe out fully, then in fully from a calibrated test-gas mixture, hold ten seconds and exhale steadily.
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Repeated for acceptability
The manoeuvre is repeated until two technically acceptable, reproducible results are obtained.
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Report includes % predicted and KCO
The report gives absolute TLCO, per cent predicted and the transfer coefficient (KCO) corrected for alveolar volume.
What it shows
When the transfer factor is the right test.
TLCO / DLCO answers a specific question — how well the alveolar-capillary membrane transfers gas — and its pattern of change points to the underlying process.
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Interstitial lung disease
Reduced TLCO is often the earliest and most sensitive marker of interstitial lung disease.
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Emphysema (reduced TLCO + KCO)
Loss of alveolar surface area lowers both TLCO and the transfer coefficient KCO.
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Pulmonary hypertension
A disproportionately low TLCO with preserved lung volumes raises suspicion of pulmonary vascular disease.
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Alveolar haemorrhage (raised TLCO)
Free blood in the alveoli binds carbon monoxide, producing a characteristic rise in TLCO.
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Post-COVID lung disease
Reduced TLCO is one of the most common abnormalities in persistent post-COVID respiratory symptoms.
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Chemotherapy pneumonitis
Bleomycin, immunotherapy and other agents can cause a progressive fall in TLCO — a monitored endpoint.
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Reduced with anaemia (corrected)
Anaemia reduces the measured value; the report gives a haemoglobin-corrected TLCO.
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Red flag: rapid TLCO decline in ILD — urgent ILD-MDT referral
A rapid drop in transfer factor in known interstitial lung disease warrants urgent ILD-MDT review.
Next steps
What follows a low or falling transfer factor.
The pattern of TLCO change guides the pathway — from reassurance through anti-fibrotic therapy, oxygen, rehabilitation, and specialist referral.
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Reassurance if normal
A normal, reproducible transfer factor with normal spirometry is genuinely reassuring.
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Anti-fibrotic therapy for IPF
Nintedanib or pirfenidone slow the fall in lung function in idiopathic pulmonary fibrosis.
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Oxygen therapy if hypoxic
Ambulatory or long-term oxygen where resting or exertional hypoxia is confirmed.
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Pulmonary rehabilitation
Structured exercise and education programme with a measurable effect on breathlessness and quality of life.
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Bronchodilators for COPD
Long-acting bronchodilators, with inhaled steroids where indicated, in emphysema-predominant COPD.
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Immunosuppression for CTD-ILD
Mycophenolate or rituximab under specialist care in connective-tissue-disease-related interstitial lung disease.
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Cardiology referral for PH
Suspected pulmonary hypertension is referred to a specialist pulmonary vascular service for right-heart catheterisation.
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Structured respiratory follow-up
Serial TLCO, spirometry and imaging on a defined interval to detect progression early.
Red flags
When gas transfer results demand urgent action.
These patterns warrant same-week specialist review — often via a respiratory or ILD multidisciplinary team.
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Rapid TLCO decline
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Pulmonary hypertension features
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Post-COVID persistent hypoxia
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Idiopathic pulmonary fibrosis
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Connective tissue disease ILD
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Chemotherapy pneumonitis
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Alveolar haemorrhage
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Chronic hypersensitivity pneumonitis
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Systemic sclerosis-related lung disease
Frequently asked
Everything patients ask about gas transfer.
Quick answers on TLCO vs KCO, what raises and lowers the transfer factor, why smoking matters and what the test actually feels like.
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What is a gas transfer test?
The gas transfer test — transfer factor for carbon monoxide, TLCO or DLCO — measures how efficiently the lungs move gas from the alveolar air spaces into the pulmonary capillary blood. It is a single-breath, ten-second manoeuvre performed in a lung function laboratory.
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What is the difference between TLCO and KCO?
TLCO is the total transfer factor across the whole lung. KCO is the transfer coefficient — TLCO divided by the alveolar volume. KCO tells you about the efficiency of gas transfer per unit of lung actually involved; the two together separate loss of surface area (emphysema) from vascular disease.
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Why can’t I smoke before the test?
Recent smoking raises carboxyhaemoglobin — carbon monoxide already bound to your haemoglobin — which artificially lowers the measured transfer factor. Departments ask for at least four hours without smoking so the result reflects your lungs, not last hour’s cigarette.
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What conditions reduce TLCO?
Interstitial lung disease, emphysema, pulmonary hypertension, chronic pulmonary embolism, chemotherapy-related pneumonitis and severe anaemia all reduce measured TLCO. In interstitial lung disease it is often the earliest abnormality to appear.
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What conditions raise TLCO?
Alveolar haemorrhage classically raises TLCO because free blood in the alveoli binds carbon monoxide. Acute asthma, polycythaemia, obesity and left-to-right cardiac shunts can also increase the value.
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Is the test safe?
The test is very safe. The inhaled gas mixture contains a trace of carbon monoxide, a trace of a marker gas such as methane, and air — well below any exposure threshold. There is no radiation and no injection. The only common effect is transient light-headedness from the breath-hold.
Sources
The guidelines behind this page.
Reviewed against current British Thoracic Society, ERS/ATS and ARTP standards. Next scheduled review: 2027-07-30.
- British Thoracic Society. Interstitial lung disease guideline.
- European Respiratory Society / ATS. Standardisation of the single-breath determination of carbon monoxide uptake in the lung (TLCO).
- American Thoracic Society. Pulmonary function testing standards.
- Association for Respiratory Technology and Physiology (ARTP). Practical handbook of pulmonary function testing.
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In practice, in London
Booking gas transfer privately in London — what actually happens
With gas transfer, the London question is usually about report turnaround and the radiologist reading it — not whether the scan is available. Waiting lists on the NHS for gas transfer vary widely by borough and by how the GP letter reads. Privately in London, we can normally offer a slot inside the same week, sometimes within 48 hours if there’s a cancellation. The difference isn’t clinical quality — the consultants are frequently the same faces you’d see on the NHS — it’s the calendar.
In practice, a private gas transfer appointment in London means a named consultant, a proper hour in the room (or the equivalent on a video call), and a report you can actually read. Most of the imaging suites and endoscopy units we use sit within a mile of Harley Street or in Chelsea and Fulham, and turnaround on findings is measured in days, not weeks. For gas transfer specifically, the difference between a routine report and a sub-speciality read is where private care earns its keep.
Where a good concierge earns its keep is in the matching. There are dozens of consultants in London who see gas transfer — but not all of them are the right fit for every case. We narrow it down based on subspecialty, insurer coverage, the specific question being asked, and whether continuity into treatment matters. The right first appointment saves you from repeating yourself later.
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