Diffusing capacity

Alternative names
Lung diffusion testing; Diffusion capacity; DLCO test

Lung diffusion testing is used to determine how well oxygen passes from the air sacs of the lungs into the blood.

How the test is performed
You inhale a single breath from a volume of gas containing a known small quantity of carbon monoxide, hold your breath for 10 seconds, then rapidly exhale. The exhaled gas is then analyzed to determine the amount of carbon monoxide that was absorbed during the breath.

How to prepare for the test
Do not eat a heavy meal before the test. Do not smoke for at least 4 to 6 hours prior to the test. Specific instructions will be given if bronchodilators or inhaler medications should not be used before the test.

For infants and children:
The preparation you can provide for this test depends on your child’s age and experience. For specific information regarding how you can prepare your child, see the following topics:

  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years) How the test will feel There is a tight fitting mouth piece to breathe through, and nose clips are applied. Why the test is performed The test is performed to determine the presence and extent of certain lung diseases that affect how well gases cross from the air sacs into the bloodstream. Normal Values This test is usually reported as the percent of predicted amount of carbon monoxide inhaled that should be absorbed, according to the age, sex, and height of the person. What abnormal results mean Abnormal results generally indicate that gases do not diffuse normally across lung membranes. This may indicate that certain lung diseases are present. Some of these diseases are diffuse interstitial fibrosis, sarcoidosis, asbestosis, and emphysema. What the risks are There are no significant risks. Special considerations Other pulmonary function tests may be done in combination with this test.

    Johns Hopkins patient information

    Last revised: December 7, 2012
    by Sharon M. Smith, M.D.

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