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Introduction ⬇

Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO) Test

Diffusing capacity of the lungs for carbon monoxide (DLCO) test, also known as transfer factor of the lung for carbon monoxide (TLCO), is a measurement that determines the rate of gas transfer across the alveolar capillary membranes. Carbon monoxide (CO) combines with hemoglobin about 210 times more readily than does O2. If there is a normal amount of hemoglobin in the blood, the only other significant limiting factor to CO uptake is the state of the alveolar capillary membranes. Normally, the amount of CO in the blood is insufficient to affect the test. Two categories of factors (i.e., physical and chemical) determine the rate of gas (CO) transfer across the lung. The physical determinants are CO driving pressure, surface area, thickness of capillary walls, and diffusion coefficient for CO. The chemical determinants are red blood cell volume and reaction rate with hemoglobin.

This test is used to diagnose pulmonary vascular disease, emphysema, and pulmonary fibrosis and to evaluate the extent of functional pulmonary capillary bed in contact with functional alveoli. The alveolar volume (VA) can also be determined. The DLCO measures the diffusing capacity of the lungs for CO. The diffusing capacity of the lungs for O2 (DLO2) is obtained by multiplying the DLCO by 1.23.

Procedure ⬆ ⬇

  1. Have the patient either sit or stand. Place nose clips on the nose and instruct the patient to breathe normally through a mouthpiece/filter (bacterial/viral) combination into the diffusion instrument.

  2. Ask the patient to expire maximally and then inspire maximally (a diffusion gas mixture), hold breath for 10 seconds, and then exhale, at which time a sample of exhaled gas is obtained.

  3. Two techniques are used by pulmonary laboratories:

    1. Single-breath or breath-holding technique

    2. Steady-state technique

  4. See Chapter 1 guidelines for intratest care.

Clinical Implications ⬆ ⬇

  1. Decreased values are associated with:

    1. Multiple pulmonary emboli

    2. Emphysema

    3. Lung resection

    4. Pulmonary fibroses:

      1. Sarcoidosis (abnormal collection of inflammatory cells in multiple organs)

      2. Systemic lupus erythematosus (SLE)

      3. Asbestosis

      4. Pneumonia

    5. Anemia

    6. Increased levels of carboxyhemoglobin (COHb)

    7. Pulmonary resection

    8. Scleroderma

  2. Increased values are observed in polycythemia, left-to-right shunts, pulmonary hemorrhage, and exercise.

  3. The value is relatively normal in chronic bronchitis.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose and procedure. Assess for interfering factors and explain that this noninvasive test requires patient cooperation.

  2. Assess the patient’s ability to follow verbal instructions.

  3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed.

  2. Discuss with the patient the possible need for follow-up testing to monitor course of therapy.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

Exercise (with an increased cardiac output) and polycythemia increase the value. Because increased levels of COHb (as seen in smokers) and anemia decrease the value, the DLCO is adjusted for COHb levels >10% (>0.10) and hemoglobin (Hb) values <8 g/dL (<80 g/L).

Reference Values ⬆

Normal