94729

Diffusing capacity of the lungs (eg, carbon monoxide, membrane diffusing capacity)

CPT code 94729 describes the measurement of the diffusing capacity of the lungs, a physiological test typically referred to as DLCO (Diffusing Capacity of the Lung for Carbon Monoxide). This test is a vital component of comprehensive pulmonary function testing (PFT) and is used to evaluate how effectively gases, such as oxygen, transfer from the inhaled air in the lung's alveoli into the red blood cells within the pulmonary capillaries. Carbon monoxide (CO) is used as the test gas because it has an extremely high affinity for hemoglobin, making it an ideal surrogate for oxygen to measure gas transfer efficiency without being limited by pulmonary blood flow. During the procedure, the patient is required to exhale to residual volume and then rapidly inhale a specialized gas mixture containing a precise, low concentration of carbon monoxide, a tracer gas (such as helium, neon, or methane), oxygen, and nitrogen. The patient then holds their breath for approximately 10 seconds at total lung capacity. During this breath-hold period, CO diffuses across the alveolar-capillary membrane and binds to hemoglobin. The patient then exhales, and a sample of the 'alveolar' portion of the exhaled gas is collected and analyzed. By comparing the concentration of the tracer gas and CO in the inhaled versus exhaled air, the clinician can calculate the amount of CO absorbed and the alveolar volume (VA). This allows for the determination of the DLCO and the DLCO/VA ratio (transfer coefficient or KCO). A decrease in DLCO indicates a reduction in the surface area available for gas exchange or an increase in the thickness of the blood-gas barrier, common in conditions like emphysema, pulmonary fibrosis, and pulmonary hypertension.

Clinical Indications

  • Evaluation of unexplained dyspnea (shortness of breath)
  • Differentiation between obstructive lung diseases (e.g., asthma vs. emphysema)
  • Monitoring of interstitial lung diseases (ILD) such as idiopathic pulmonary fibrosis
  • Screening for pulmonary involvement in systemic diseases like scleroderma or sarcoidosis
  • Assessment of lung toxicity from medications (e.g., amiodarone, bleomycin, methotrexate)
  • Pre-operative evaluation prior to lung resection or major thoracic surgery
  • Evaluation of pulmonary vascular disease (e.g., pulmonary arterial hypertension)
  • Monitoring the effect of cigarette smoking or occupational exposures on lung function
  • Assessment of disability for occupational or insurance purposes

Procedure Steps

  1. The patient is seated comfortably and fitted with a nose clip to prevent nasal breathing.
  2. The equipment (diffusing capacity system) is calibrated for gas concentrations and volume.
  3. The patient breathes normally through a mouthpiece connected to the testing apparatus.
  4. The patient is instructed to exhale completely to residual volume (RV).
  5. The patient rapidly inhales the test gas mixture (containing CO and a tracer gas) to total lung capacity (TLC).
  6. The patient holds their breath for 10 seconds (+/- 2 seconds) while maintaining a closed glottis.
  7. The patient exhales rapidly and smoothly.
  8. A specific 'alveolar' sample of the exhaled gas is captured after the initial dead-space air is discarded.
  9. The concentrations of CO and the tracer gas in the exhaled sample are analyzed by the system sensors.
  10. The DLCO value is calculated, often adjusted for the patient's current hemoglobin level and carboxyhemoglobin levels if available.
  11. The test is typically repeated after a rest period (usually 4-5 minutes) to ensure reproducibility, with the average of at least two acceptable maneuvers reported.

Coding Guidelines

  • CPT 94729 is an add-on code and must be reported in conjunction with a primary pulmonary function test code.
  • Common primary codes used with 94729 include 94010 (Spirometry), 94060 (Bronchodilation responsiveness), 94070 (Bronchospasm provocation), or 94375 (Respiratory flow volume loop).
  • This code should be reported once per session, regardless of the number of trials or maneuvers performed to obtain the result.
  • If the test is performed to measure membrane diffusing capacity (DM) and capillary blood volume (Vc) specifically, 94729 is still the appropriate code used to describe the diffusing capacity component.
  • Modifier 26 should be appended if only the professional component (interpretation) is being billed by the physician.
  • Modifier TC should be appended if only the technical component (equipment and staff) is being billed by the facility.
  • Do not report 94729 in conjunction with codes that already bundle gas exchange analysis if specified by NCCI edits.

Associated ICD-10 Codes