DlcoEdit

DLCO, short for the diffusing capacity of the lung for carbon monoxide, is a cornerstone measurement in pulmonary function testing. By quantifying how readily gas moves from the alveolar air into the blood, DLCO provides a practical readout of the integrity of the alveolar-capillary interface and the volume of usable pulmonary capillaries. It is routinely used to help diagnose, stage, and monitor lung diseases, and it plays a role in evaluating surgical risk before certain procedures. diffusing capacity of the lung for carbon monoxide is typically reported as a percentage of a person’s predicted value based on age, sex, height, and other factors. The test can be performed using different methods, most commonly the single-breath technique, and results are influenced by several physiological and technical variables.

DLCO is not a stand-alone diagnosis; it is best interpreted in the context of other information from a full clinical assessment, including symptoms, imaging, and other pulmonary function tests such as spirometry and lung volumes. The value reflects two main components: the surface area and integrity of the alveolar-capillary membrane, and the capillary blood volume available for gas transfer. Because carbon monoxide binds with high affinity to hemoglobin, DLCO also depends on the amount of hemoglobin in the blood. For this reason, clinicians often adjust DLCO values for hemoglobin levels, producing a corrected value sometimes referred to as DLCOc. When interpreting DLCO, it is common to consider the alveolar volume (VA) measured during the test, since reduced VA can lower the observed DLCO independent of membrane diffusion.

Overview

What it measures

DLCO assesses the efficiency of gas transfer from the lungs into the bloodstream. It is particularly sensitive to diseases that affect the alveolar walls, the capillary bed, or the volume of accessible capillaries. Conditions that stiffen or thicken the alveolar-capillary membrane or reduce the surface area can lower DLCO, while conditions that increase pulmonary capillary blood volume can raise it.

How it is measured

The most common method is the single-breath technique, in which the patient inhales a test gas mixture containing a small amount of carbon monoxide, holds the breath for about ten seconds, and then exhales. The uptake of carbon monoxide during this brief interval provides an index of diffusion capacity. Other methods exist, including steady-state approaches, but the single-breath DLCO remains the standard in many labs. Results are typically reported as a percent of predicted value and may be reported alongside DLCO corrected for hemoglobin (DLCOc) and DLCO adjusted for alveolar volume (DLCO/VA or KCO, the transfer coefficient).

Interpretation and related indexes

Normal ranges vary by lab, age, sex, and height, but a common target is roughly 80–120% of predicted for DLCO. Values below this range point to diffusion impairment, whereas values above the normal range are uncommon and often reflect measurement variability or high pulmonary blood volume in certain circumstances.
The DLCO/VA ratio (sometimes termed KCO) helps distinguish causes of a low DLCO. A low DLCO with a low VA often indicates a diffusion problem relative to lung volume, while a low DLCO with a normal or high VA can point to issues with the alveolar-capillary membrane or blood volume rather than reduced lung size alone.
Hemoglobin correction is important because anemia lowers DLCO by reducing the available binding sites for carbon monoxide. Adjusting for Hb helps prevent misclassification of diffusion capacity.

Clinical uses

Diagnostic support in suspected interstitial lung disease idiopathic pulmonary fibrosis, other forms of interstitial lung disease and diseases that affect the alveolar-capillary membrane.
Evaluation of obstructive diseases where diffusion capacity is reduced, most notably emphysema emphysema and related chronic obstructive conditions.
Preoperative risk assessment for thoracic and non-thoracic surgery, to gauge the likelihood of postoperative pulmonary complications.
Monitoring disease progression or response to therapy in certain conditions that affect the alveolar-capillary interface or pulmonary blood volume.
Assessment of lung involvement in systemic diseases with pulmonary manifestations.
Screening for anemia-related reductions in gas transfer when there is clinical concern about respiratory impairment.

Factors that influence DLCO

Hemoglobin level: anemia lowers DLCO; polycythemia can raise it. Corrected DLCO values help mitigate this effect.
Alveolar volume: smaller lung volumes can reduce DLCO even if the diffusion barrier is normal.
Smoking and tobacco exposure: smoking can acutely affect DLCO readings; smoking status should be considered when interpreting results.
Altitude and environmental factors: testing conditions can alter absolute DLCO values, so predicted values are lab-specific.
Pregnancy, recent exercise, and acute illness can transiently influence measurements.
Calibration and technique: DLCO is sensitive to coaching, breath-holding, and patient effort; inter-lab variability exists, underscoring the need for standardized procedures and quality control.

Limitations

DLCO is a surrogate measure and must be integrated with a broader clinical picture.
It does not pinpoint the exact cause of diffusion impairment; imaging, history, and other tests are often required to establish a diagnosis.
The test is effort-dependent and may be influenced by hemodynamic or hematologic factors not related to lung parenchyma.
Lab-to-lab variability and reliance on predicted values can complicate comparisons across institutions or over time if standardization is not maintained.

Controversies and debates

From a practical, resource-conscious perspective, there is ongoing discussion about when DLCO testing provides added value and how often it should be used in routine practice. Some arguments emphasize that DLCO should be reserved for cases where test results are likely to influence management, such as preoperative risk stratification, diagnostic clarification when other tests are inconclusive, or monitoring of diseases known to affect gas transfer. Critics of broader, blanket testing argue that unnecessary measurements add cost and complexity without improving patient outcomes in many routine cases. Proponents of targeted testing counter that DLCO can reveal subtle disease involvement earlier, potentially guiding therapy decisions before symptoms become prominent, especially in diseases with high morbidity.

Another area of debate concerns standardization and interpretation. Because DLCO depends on hemoglobin, alveolar volume, and testing technique, laboratories must adhere to rigorous quality control and clear reporting standards. Some practitioners advocate for broader adoption of hemoglobin-corrected DLCO and for explicit reporting of DLCO, DLCOc, and DLCO/VA (KCO) to aid interpretation. Others caution that the additional data can complicate decision-making if clinicians are not comfortable integrating the multiple indices. The balance between precision and practicality remains a central theme in discussions about how best to implement DLCO testing in diverse clinical settings.

Finally, there is discussion about how DLCO data fit into evolving models of value-based care and guideline development. While DLCO can be highly informative in certain clinical scenarios, opponents of over-testing argue for using high-quality evidence to define clear thresholds for when DLCO testing alters management. Supporters of more proactive testing point to patient outcomes and resource allocation, arguing that better diagnostic clarity can prevent downstream costs from misdiagnosis and delayed treatment. In this debate, the emphasis remains on delivering effective care while avoiding waste and unnecessary interventions.