Diffusing capacity for carbon monoxide


DLCO or TLCO is the extent to which oxygen passes from the air sacs of the lungs into the blood. Commonly, it refers to the test used to determine this parameter. It was introduced in 1909.

Mechanism

This test involves measuring the partial pressure difference between inspired and expired carbon monoxide. It relies on the strong affinity and large absorption capacity of red [blood cell]s for carbon monoxide and thus demonstrates gas uptake by the capillaries that are less dependent on cardiac output. The measurement of DLCO is affected by atmospheric pressure and/or altitude and correction factors can be calculated using the method recommended by the American Thoracic Society. Expected DLCO is also affected by the amount of hemoglobin, carboxyhemoglobin, age and sex. The correction for hemoglobin is based on the method of Cotes as recommended by the American Thoracic Society.

DLCO vs TLCO

Generally DLCO is measured in "ml/min/kPa" and TLCO is measured in "mmol/min/kPa".

Factors affecting DLCO

Decrease

DLCO is decreased in any condition which affects the effective alveolar surface area:
  1. Hindrance in the alveolar wall. e.g. fibrosis, alveolitis, vasculitis
  2. Decrease of total lung area, e.g. Restrictive lung disease or lung resection.
  3. Chronic obstructive pulmonary disease due to decreased surface area in the alveoli, as well as damage to the capillary bed
  4. Pulmonary embolism
  5. Cardiac insufficiency
  6. Pulmonary hypertension
  7. Bleomycin
  8. Anemia-due to decrease in blood volume
  9. Amiodarone high cumulative dose; more than 400 milligrams per day
  10. After chemotherapy and radiotherapy
However, many modern devices compensate for the hemoglobin value of the patient, and excludes it as a factor in the DLCO interpretation.

Increase

Factors that can increase the DLCO include polycythaemia, asthma and increased pulmonary blood volume as occurs in exercise. Other factors are left to right intracardiac shunting, mild left heart failure and alveolar hemorrhage.

Significance of results

There is no universally recognized reference value range for DLCO as of 2017, but values in the 80%-120% of predicted range based on instrument manufacturer standards are generally considered normal. A DLCO of less than 60% predicted portends a poor prognosis for lung cancer resection. FEV1 is of lesser prognostic value for lung resection survival.