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Lung function

Gas transfer test (TLCO)

The breathing test that measures how well oxygen actually crosses from your lungs into your blood.

Key facts

At a glance

  • The method: one breath of a harmless test gas mixture, held for about ten seconds, then exhaled for analysis.

  • What it detects: damage to the lung’s gas-exchange surface - from emphysema, fibrosis or blood vessel disease - often before other tests change.

  • Preparation: no smoking on the day (it directly distorts results), and follow instructions on inhalers and exertion.

  • Results: reported with full lung function data, typically within days privately.

  • Follow-up: a respiratory physician interprets the value - correction factors for anaemia and smoking are built into good reports.

The full picture

What this test involves, and how to get it

Gas transfer - TLCO or DLCO on reports - measures the business end of breathing: not how much air moves, but how well oxygen crosses the membrane from air sac to bloodstream. You inhale a single breath of a dilute test gas mixture containing a trace of carbon monoxide, hold it for around ten seconds, and exhale; the analyser calculates how much was taken up. The result probes the integrity and area of the gas-exchange surface, which is why it falls in emphysema (destroyed air sacs), pulmonary fibrosis (thickened membranes) and pulmonary hypertension (reduced blood flow) - sometimes while spirometry and X-rays still look reassuring. It is also tracked serially in people on drugs that can affect the lungs.

Interpretation has honest nuances: anaemia lowers the reading and recent smoking distorts it, so good laboratories correct for haemoglobin and insist on no cigarettes beforehand; a low TLCO with normal spirometry is a specific and useful clue pointing towards vascular or early interstitial disease. TLCO is performed as part of full lung function testing in hospital laboratories - NHS access follows respiratory referral with variable waits, while private lung function labs offer the test within one to two weeks, reported alongside volumes and spirometry and reviewed with a respiratory physician who can place your number in the pattern that actually identifies the disease.

A quiet reminder

This guide is for information, not medical advice.

Your own clinician knows your history and can tell you which parts apply to you. Next review of this page: 2027-09-27.

Frequently asked

Common questions about Gas transfer test (TLCO).

Quick, plain-English answers to what people ask us most.

  • What is a gas transfer test (TLCO)?

    This lung function test measures how efficiently oxygen and carbon dioxide move between the tiny air sacs in your lungs and your bloodstream. You breathe in a small amount of a safe tracer gas, and the machine measures how much of it transfers into your blood.

  • Why would a doctor order a gas transfer test?

    It's used to investigate breathlessness that isn't fully explained by other tests, and to help diagnose and monitor conditions affecting the lung tissue itself, such as pulmonary fibrosis, or conditions affecting blood vessels in the lungs. It's often done alongside spirometry and lung volume measurements.

  • Do I need to prepare for it?

    You may be asked to avoid smoking and strenuous exercise for a short period before the test, as these can affect results. Your clinic will provide specific instructions when booking.

  • What happens during the test?

    You'll breathe through a mouthpiece, taking a single deep breath of a gas mixture and holding it briefly before breathing out, sometimes repeated a couple of times for accuracy. The whole test usually takes around 15 to 20 minutes.

  • Does it hurt or have risks?

    The test is painless and the gas used is in a very small, safe concentration. Some people find the breath-holding part briefly uncomfortable, but there are no significant risks.

  • How long do results take?

    A preliminary reading is usually available at the time of testing, with a formal specialist report typically following within a few days.

  • What do abnormal results mean, and what happens next?

    A reduced gas transfer value can point to conditions affecting the lung tissue or blood vessels, and the pattern of results helps guide further diagnosis alongside your symptoms and other tests. Your doctor will discuss the findings and may recommend imaging or referral to a respiratory specialist.

  • How much does it cost privately, and is it available on the NHS?

    Gas transfer testing is available on the NHS through respiratory departments, usually via specialist referral, though waits for non-urgent testing can be lengthy. Private testing is typically available within one to two weeks, with specialist interpretation, and cost varying by provider.

  • Is it accurate and reliable?

    This is a well-established, standardised test used widely in respiratory medicine, though results depend on good technique and can be affected by factors like anaemia, which your doctor will take into account. It's most useful when interpreted alongside other lung function measurements.

  • How do I book it privately, and how quickly can I be seen?

    You can book a private gas transfer test, often as part of a broader respiratory assessment, with appointments typically available within one to two weeks.