Lung Diffusion Testing

Written by Krista O'Connell | Published on June 1, 2012
Medically Reviewed by George Krucik, MD

What is Lung Diffusion Testing?

From asthma to chronic obstructive pulmonary disease (COPD), there are a variety of conditions that can affect the lungs. Wheezing or general shortness of breath may be signs that the lungs are not functioning exactly as they should be. When patients exhibit signs of lung problems, doctors may order tests to assess lung function.

A lung diffusion test examines how your lungs are processing air. Along with other tests, it can help your doctor determine how your respiratory system is functioning.

What is Lung Diffusion?

Lung diffusion testing is designed to test the efficiency of your lungs in changing air into oxygenated blood. This process is called diffusion.

When we breathe, we inhale air containing oxygen through the nose and mouth. This air travels down the trachea (windpipe) into the lungs. Once in the lungs, the air travels through a series of smaller and smaller structures. It eventually reaches tiny sacs called alveoli.

From the alveoli, the oxygen enters the blood in nearby vessels. This is oxygen diffusion. Once the blood is oxygenated, it carries this oxygen throughout the body.

Another form of diffusion is when blood containing carbon dioxide travels back to the lungs. The carbon dioxide is taken from the blood and expelled through exhalation. This is carbon dioxide diffusion.

Lung diffusion testing can analyze both oxygen and carbon dioxide diffusion.

The Purpose of Lung Diffusion Testing

Lung diffusion testing is typically used by doctors to assess people who have lung disease or to help diagnose such diseases. These diseases include:

  • emphysema
  • pulmonary hypertension (high blood pressure in the arteries of the lungs)
  • sarcoidosis (inflammation of the lungs)

If you show symptoms of lung disease, this is one test that may be used to analyze how your lungs are functioning. Also, if you are undergoing treatment for lung disease, your doctor may order this test from time to time to monitor the diseases progress and how treatment is working.

What to Expect During a Lung Diffusion Test

Before the Test

Your doctor may advise you to take certain measures to prepare for a lung diffusion test. These instructions typically include:

  • not using a bronchodilator or other type of inhaled medication prior to testing
  • avoiding eating a large amount of food before the test
  • not smoking for several hours before the test

Test Steps

  • A mouthpiece will be placed around your mouth. It will fit snugly. Your doctor will place clips on your nose to prevent you from breathing through your nostrils.
  • You will take a breath of air. This air will have a tiny (and safe) amount of carbon monoxide.
  • You will hold this air for a count of 10.
  • You will quickly exhale the air you are holding in your lungs.
  • This air will be collected and analyzed.

Potential Risks of a Lung Diffusion Test

This is a very safe and straightforward procedure. A lung diffusion test does not involve any serious risk. It is a quick procedure and should not cause most people any significant pain or discomfort.

Most likely, there will not be any adverse side effects after the test is complete.

Understanding Your Test Results

Tracer Gas

This test looks at how much of a certain gas is inhaled and how much is present in the air you exhale. Usually, the lab will use carbon monoxide (or another “tracer” gas) to determine your diffusion capacity.

The lab will consider two things when determining the results of the test: the amount of carbon monoxide that was originally in the inhaled air and the amount in the air that was exhaled.

If there is far less carbon monoxide in the exhaled sample, it indicates that a large amount diffused from the lungs into the blood. This is a sign of robust lung function. If the amount in the two samples is similar, the diffusing capacity of the lungs is limited.

Mitigating Factors

A doctor will need to consider a number of factors to decide whether results suggest problems with lung function. Factors include:

  • emphysema
  • whether the patient is a man or a woman
  • the patient’s age
  • how tall the patient is
  • the amount of hemoglobin in the patient’s blood

Potential Causes of Abnormal Results

If a doctor determines that diffusion is not occurring at the level it should be, there may be a number of causes. These include:

  • emphysema
  • lung tissue loss or severe scarring
  • problems with arterial blood flow
  • a blocked artery in the lung (pulmonary embolism)
  • elevated blood pressure in the lung’s arteries
  • hemorrhage in the lung
  • inflammation

Additional Common Lung Function Tests

If your doctor suspects your lungs are not working properly, he or she may order several tests in addition to the lung diffusion test. One is spirometry. This measures the amount of air intake and how fast you can exhale it.

Another test, lung volume measurement, determines lung size and capacity. The combined results can help your doctor better figure out what is wrong and what measures can be taken to provide symptom relief.

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