Diffusion testing (synonyms: DLCO test; diffusion capacity test; CO diffusion capacity; carbon monoxide transfer factor test) is a diagnostic procedure used in pulmonology (lung medicine) to assess diffusion capacity (DLCO). However, the determination of diffusing capacity is considered to play a minor role in the diagnosis of asthma, since diffusing capacity itself is usually normal or slightly increased. Even in the case of a clear manifestation of bronchial asthma with an existing reduction of the “forced one-second capacity (FEV1)”, a still normal diffusion capacity can occur. However, the procedure is of crucial importance for the functional differential diagnosis between bronchial asthma, chronic obstructive pulmonary disease (COPD), and emphysema (irreversible hyperinflation of the smallest air-filled structures (alveoli, alveoli) of the lungs), because COPD and emphysema are associated with a decrease in DLCO.
Indications (areas of application)
- Bronchial asthma – to be able to differentiate bronchial asthma from other chronic lung diseases such as COPD, the diffusion test can be used, since a precise delimitation is possible via this procedure.
- Chronic obstructive pulmonary disease (COPD) – the diffusion test represents a part of the standard diagnostics in COPD. The diffusion capacity for carbon monoxide is reduced depending on the severity primarily of emphysema. Thus, emphysema as a component of COPD represents the main reason for the deterioration of diffusion capacity. However, the diffusion test must take into account that other components of COPD, such as the presence of chronic obstruction (narrowing) of the airways, cannot be adequately assessed as well. Based on this, further diagnostic procedures are necessary to determine the extent of COPD.
- Pulmonary emphysema – emphysema represents irreversible (irreversible) overinflation of the alveoli (smallest air-filled vesicles) of the lungs. Emphysema itself represents the common endpoint of various chronic lung diseases, including COPD.
- Sarcoidosis – sarcoidosis is a systemic disease of connective tissue that may be associated with lung involvement and may lead to granuloma formation (tissue neoplasm). In the presence of this disease with lung involvement, a reduction in diffusion capacity can typically be seen.
Contraindications
There are no relevant contraindications to performing the diffusion test.
Before the test
Measurement of the diffusing capacity of the lungs is part of the standard procedures for pulmonary function diagnosis. However, before the procedure is performed, it is important that other diagnostic procedures be used to increase the sensitivity (percentage of diseased patients in whom the disease is detected by use of the procedure, i.e., a positive finding occurs) and specificity (likelihood that actually healthy individuals who do not have the disease in question will also be detected as healthy by the procedure) of the diagnosis. Additional standard pulmonary function diagnostic procedures include spirometry and body plethysmography.
The procedure
Several methods are available to perform the procedure:
- Steady-state method – In this method, a gas mixture consisting of air and carbon monoxide is inhaled by the patient over several minutes until a steady state (equilibrium between carbon monoxide intake and output) is reached. Using a combined measurement of the respiratory volumes and the carbon monoxide concentrations, it is possible to determine the intake of carbon monoxide per minute. However, in order to obtain meaningful measurement results, it is necessary that there is uniform ventilation of all lung sections. As a disadvantage of the method is that the time required for the implementation is relatively high.
- Single-breath method – In contrast to the steady-state method, in the single-breath method, the patient inhales a gas mixture containing 0.3% carbon monoxide and 10% helium at his or her complete vital capacity. The patient must then hold their breath for ten seconds.During the subsequent exhalation, the first 750 ml of exhaled air is discarded due to existing contamination with the gas mixture from the dead space of the lung (space of the respiratory system that is not involved in the gas exchange of the lung, but serves to transport the inhaled gas mixture). The following 600-900 ml of exhaled air is analyzed. By determining the carbon monoxide and helium concentrations, the initial carbon monoxide concentration in the alveolar space and the concentration absorbed in the lungs are calculated. The quality of the results depends, among other things, on the achievement of the maximum individual total lung capacity.
- Intra-breath method – This method is characterized by the fact that only a short breath-holding time is required to perform it, making it suitable for patients who would not be able to use the single-breath method. Diffusing capacity is assessed in the intra-breath method via multiple determinations of the concentrations of the gases in the exhaled air.
After the examination
Following the performance of the procedure, no special measures are to be carried out.
Possible complications
Ingestion of carbon monoxide at this concentration is completely safe. However, it should be noted that standardized measurement conditions must be maintained to avoid artificial changes in the ventilation/perfusion ratio.
