Speaker's Highlight

• Don Sin University of British Columbia, St. Paul Hospital (Canada)
  Kenneth R. Chapman Toronto General Hospital Research Institute (Canada)
• Parameswaran Nair McMaster University (Canada)
  Carolyn Calfee UCSF (U.S.A.)
• Gregory P. Downey University of Colorado School of Medicine (U.S.A.)
  David A. Schwartz University of Colorado School of Medicine (U.S.A.)
• Neil Schluger Tuberculosis Control Branch, California Department of Public Health (U.S.A.)
  Nick Kim Critical Care & Sleep Medicine, University of California San Diego (U.S.A.)
• Nicola Hananiah Baylor College of Medicine (U.S.A.)
  Jae-Joon Yim Seoul National University College of Medicine (Republic of Korea)
• Koichiro Asano Tokai University School of Medicine (Japan)
  Diahn-Warng Perng Taipei Veterans General Hospital (Taiwan)
• Konstantinos Kostikas University of Ioannina (Greece)
  Karin Klooster University Medical Center Groningen (Kingdom of the Netherlands)

• Carbon monoxide diffusing capacity in interstitial lung disease

  Carbon monoxide diffusing capacity, commonly reported as DLCO or TLCO, is a key pulmonary function measurement in patients with interstitial lung disease (ILD). It estimates how effectively oxygen can move from the air sacs into the pulmonary circulation, offering information that spirometry and lung volumes cannot provide alone.

  Fibrotic disorders such as idiopathic pulmonary fibrosis, connective tissue disease-associated ILD, hypersensitivity pneumonitis, and sarcoidosis can impair this gas transfer in different ways. A reduced result may reflect thickened alveolar walls, loss of functioning capillary surface, pulmonary vascular disease, emphysema, or a combination of these processes.

  The value is greatest when interpreted alongside symptoms, high-resolution computed tomography, exercise oxygenation, and serial pulmonary function tests. Respiratory specialists discussing these issues at meetings connected with APSR and KATRD often emphasize that DLCO is a clinical trend, rather than an isolated label of disease severity.

  What the measurement captures

  During a standard single-breath test, the patient inhales a small concentration of carbon monoxide, holds the breath briefly, and exhales into an analyzer. Carbon monoxide is used because it binds avidly to hemoglobin, allowing the test to estimate the conductance of the alveolar-capillary membrane and the amount of pulmonary capillary blood available for uptake.

  The reported DLCO is influenced by alveolar volume (VA), inspired volume, hemoglobin concentration, carboxyhemoglobin, and the quality of the breathing maneuver. Some laboratories also report KCO, or DLCO divided by VA. KCO can add context, but it should not be treated as a simple correction for a low lung volume.

  A technically poor effort, inadequate breath holding, air leakage, recent smoking, or uneven ventilation can distort the result. Before attributing a low diffusion capacity to progressive fibrosis, clinicians check test acceptability and compare the measurement with previous studies performed under similar conditions.

  Why interstitial disease lowers gas transfer

  In ILD, inflammation and fibrosis may expand the distance that oxygen must cross between alveolar gas and red blood cells. Scarred regions may also lose capillaries and functioning alveoli, reducing the surface area available for exchange. These changes commonly produce a restrictive ventilatory pattern alongside impaired diffusion.

  A low DLCO can be especially pronounced when ILD is accompanied by pulmonary hypertension or emphysema. Pulmonary vascular remodeling reduces perfused capillary volume, while emphysema destroys alveolar walls. This is why two people with similar forced vital capacity (FVC) may have very different gas transfer results and exercise tolerance.

  Hemoglobin matters because anemia reduces the blood’s capacity to bind carbon monoxide. A hemoglobin-adjusted value can therefore provide a fairer estimate of pulmonary gas transfer, while an unadjusted value may make diffusion impairment appear more severe than it is.

  Reading a pulmonary function report

  Interpretation should begin with the reference standard used by the laboratory. The lower limit of normal and z-score are generally more informative than relying on a single percentage predicted, especially in people at the extremes of age or height. The pattern across FVC, total lung capacity, VA, DLCO, and KCO is more useful than any one number.

  A reduced DLCO with reduced TLC and FVC supports a restrictive process, although restriction should be confirmed with lung-volume testing rather than inferred from spirometry alone. A disproportionately low DLCO raises concern for pulmonary hypertension, coexisting emphysema, anemia, or extensive parenchymal disease.

  Symptoms and exertional physiology add important detail. A patient may have a moderately reduced resting DLCO but develop marked desaturation during a six-minute walk or cardiopulmonary exercise test. Conversely, a stable DLCO with worsening breathlessness may direct attention toward deconditioning, airway disease, cardiac dysfunction, infection, or another explanation.

  Patterns that change interpretation

  The clinical setting determines how much weight to give the result. In systemic sclerosis, for example, a falling diffusion capacity may signal pulmonary vascular involvement or evolving ILD. In idiopathic pulmonary fibrosis, serial decline can support disease progression, but the rate of change must be assessed against measurement variability and the broader clinical picture.

  Mixed obstructive and interstitial disease can make interpretation difficult. A smoker with emphysema and fibrosis may have relatively preserved lung volumes but severely impaired gas transfer. When breathlessness occurs during an acute respiratory deterioration, clinicians should also assess infection, pulmonary embolism, heart failure, and COPD exacerbation; acute exacerbation guidance can help frame assessment when obstructive disease is part of the history.

  Finding	Possible explanation	Useful next consideration
  Low DLCO with low TLC	Fibrotic restriction or extensive ILD	Review HRCT and serial lung volumes
  Low DLCO with near-normal TLC	Pulmonary vascular disease, emphysema, anemia, or early ILD	Check hemoglobin, imaging, echocardiography, and symptoms
  Low VA with preserved or relatively high KCO	Extrapulmonary restriction or suboptimal inhaled volume	Assess technique, chest wall factors, and neuromuscular function
  Rapid fall in DLCO	Disease progression, vascular complication, acute illness, or technical variation	Repeat testing and investigate new clinical findings
  Severe exertional desaturation	Limited pulmonary reserve or vascular impairment	Perform walk testing, oxygen assessment, and targeted cardiac evaluation

  Practical reporting priorities

  A clear report should explain whether the result is within the expected range, how it compares with earlier testing, and whether the change is clinically meaningful. A fall in DLCO is more concerning when it occurs with worsening dyspnea, declining FVC, new oxygen requirement, or radiographic progression.

  Clinicians should record factors that influence measurement quality, including smoking status, recent respiratory infection, hemoglobin level, supplemental oxygen use, and the interval between tests. Consistent laboratory methods improve the reliability of longitudinal comparisons.

  • Confirm test quality and review the laboratory’s reference equations.
  • Consider hemoglobin correction when anemia is present or suspected.
  • Interpret DLCO together with FVC, TLC, VA, KCO, imaging, and exercise oxygenation.
  • Investigate a disproportionate reduction for pulmonary hypertension or emphysema.
  • Repeat an unexpected change when technical variation or acute illness may be responsible.

  Turning results into clinical action

  DLCO helps establish baseline functional impairment, estimate respiratory reserve, and monitor response or progression over time. It can inform decisions about rehabilitation, supplemental oxygen assessment, referral for pulmonary hypertension evaluation, and the urgency of specialist review. It also contributes to preoperative and treatment-risk discussions, although it should not be used as the sole determinant of prognosis.

  For patients with connective tissue disease or suspected progressive pulmonary fibrosis, scheduled reassessment can reveal change before symptoms become dramatic. The most useful plan combines objective testing with the patient’s functional history: walking distance, recovery time, activity limitation, cough, chest discomfort, and oxygen saturation during exertion.

  Respiratory teams can strengthen care by documenting the complete diffusion-capacity context rather than recording only a percentage predicted. Review the DLCO method, correction factors, trend, imaging, and exercise response at the next pulmonary assessment so that an abnormal result leads to a focused clinical decision.

  Richard Russell Nuffield Department of Clinical Medicine, University of Oxford (United Kingdom)
• Mona Bafadhel King’s College London (United Kingdom)
  David Jackson Guy’s and St Thomas’ Hospital, King’s College London (United Kingdom)
• James Chalmers University of Dundee (United Kingdom)
  David Price University of Aberdeen (United Kingdom)