Lung function tests

Lung function tests are a useful way of determining how well the lungs are working. They are an important tool to help identify the cause of breathlessness or lung disease. Lung function tests help to determine airflow, lung capacity and the ability of the lungs to exchange oxygen and carbon dioxide between the blood and lungs. 

Tests carried out in the lung function laboratory include:

  • Spirometry
  • Body plethysmography
  • Gas transfer
  • Mouth pressures
  • Fractional exhaled nitric oxide
  • Hypoxic challenge (fitness to fly) test
  • Cardiopulmonary exercise tests
Feno test


Spirometry allows the determination of airflow limitation in the lungs. The individual will be asked to wear a clip on their nose that prevents air from escaping from the nose. Following a deep breath they are asked to blow out as quickly and as forcibly as they can into a mouth piece. 

In addition to identifying airflow limitation in your airways spirometry allows determination of obstructive and restrictive lung disease. For example, to differentiate between asthma and COPD, or interstitial lung disease, or chest wall restriction. 

Sometimes a bronchodilator such as salbutamol is administered and the spirometry test repeated 15 min later. This will allow to determine if their is significant dilatation of the airways. Significant bronchodilator reversibility as seen in asthma is an increase in forced expiratory volume in 1 second of 12% or more or more than 200 mL.

Body plethysmography

Body plethysmography is a test used to determine lung volumes. For example, it can determine your total lung capacity (ie. how much total amount of air in the lungs at the end of full inspiration) and the residual volume (ie. the total amount of air in the lungs at the end of full expiration). People with restrictive lung disease have reduced lung volumes and those with obstructive lung disease may have a increased residual volume because narrow airways cause air to be trapped in the lungs as it isn’t completely exhaled. 

    Gas transfer

    This tests is also known as transfer factor or diffusing capacity. It is a measure of how well the lungs exchange gases between themselves and the blood. To do this test a mixture containing a small amount of helium and carbon monoxide is inhaled through a deep breathe. After a breath hold of 8 to 10 seconds the air from the lungs is slowly exhaled into the machine for a reading. To get an accurate representation the value obtained needs to be corrected for the lung volume and the haemoglobin in blood as these two parameters affect the transfer factor value. A low transfer factor indicates impaired gas exchange. 

    Respiratory muscle tests

    Respiratory muscle tests are useful to determine if the breathing problem is due to weak respiratory muscles. These tests include:

    Mouth pressures

    After breathing our air from the lungs as long as possible the person is asked to suck hard on a mouthpiece for 1 second to measure the maximal inspiratory pressure. Then after taking a deep breath in the person exhales forcibly against a closed off mouthpiece to determine the maximal expiratory pressure. 

    Sniff pressures

    A small probe is inserted into one nostril and measures the inspiratory and expiratory pressures when the person sniffs.

    Lying and sitting / standing vital capacity

    Spirometry is done lying down in the supine position and then again in a sitting or standing position. A significant reducation in the supine VC may suggest respiratory muscle and diaphragmatic weakness.

    Fractional exhaled nitric oxide

    Inflammation in the lungs can produce nitric oxide and high levels of NO (>40 ppb) may suggest asthma. However not all types of asthma will produce a high level of NO. This test involves taking in a deep breath and then slowly exhaling at a constant rate into the machine that takes the reading. 

    Fitness to fly test

    This test mimics the conditions in a pressurised cabin at altitude. Although an airplane may be flying at higher altitude the cabin is pressurised to 5000 – 8000 feet where the oxygen concentration in air is reduced from 21% to 15%. For people with healthy lungs and oxygen saturations >94% this isn’t a problem. But for people with signifcant respiratory disease and oxygen saturations >95% there oxygen saturations may drop significantly during the flight leading to respiratory distress.

    To do the test the person sits in a cabin breathing air with 15% oxygen and using a pulse oximeter the oxygen saturation is monitored and is used to determine if the person needs oxygen during a flight, and if they do, how much oxygen per minute they will need. 

    Cardiopulmonary exercise test

    Cardiopulmonary exercise testing, also known as CPET or CPEX, is a specialised test that provides information on how the heart, lungs and muscles are working together during exercise. It can therefore help determine if breathlessness is caused by a problem with the heart or lungs, a combination of the two, breathing pattern disorder or lack of fitness. It is also very useful to determine fitness before major surgery. 

    For the test the person will exercise on an upright stationary bicycle or treadmill. They will breathe through a mouthpiece and recordings are taken from the breath,  from heart monitors (continuous electrocardiogram and regular blood pressure). Capillary or arterial blood gases are also taken to measure blood oxygen, carbon dioxide and lactate levels. 

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