Diffusing Capacity

Diffusing capacity, usually measured as the diffusing capacity (or transfer factor) for carbon monoxide, quantifies how efficiently gas crosses from the alveolar air into the pulmonary capillary blood. It probes the combined integrity of the alveolar-capillary membrane, the available surface area, and the volume of red cells in the pulmonary capillaries.

Definition

Pulmonary diffusing capacity (DLCO, or transfer factor TLCO) is the rate of uptake of carbon monoxide from alveolar gas per unit driving pressure, used as an index of the efficiency of gas transfer across the alveolar-capillary membrane; it is most commonly measured by the single-breath method.

Scope

This topic covers the single-breath carbon monoxide method, the physiological determinants of gas transfer (membrane, surface area, and capillary blood), the partition of diffusing capacity into membrane and blood-volume components, and the interpretation and adjustment of results (for example for hemoglobin and alveolar volume). It is a methodological reference, not clinical guidance.

Core questions

• Why is carbon monoxide used to measure gas transfer?
• What physiological factors determine the diffusing capacity?
• How are membrane and capillary-blood components distinguished?
• How should DLCO be adjusted for hemoglobin and alveolar volume?

Key concepts

• Single-breath carbon monoxide method
• Transfer coefficient (KCO, DLCO/VA)
• Membrane component and capillary blood volume
• Roughton-Forster relationship
• Hemoglobin correction
• Alveolar volume (VA)
• Diffusion limitation versus perfusion limitation

Mechanisms

Carbon monoxide is used because its uptake is limited almost entirely by diffusion rather than by blood flow, owing to its very high affinity for hemoglobin, which keeps its capillary partial pressure near zero. In the single-breath test, a subject inhales a dilute carbon monoxide mixture, holds the breath, and the rate of disappearance of carbon monoxide is measured to compute uptake per unit driving pressure. Total diffusing capacity reflects both the conductance of the alveolar-capillary membrane and the volume of capillary blood available to bind carbon monoxide; the Roughton-Forster relationship partitions these into a membrane component and a blood-volume component. Because uptake depends on hemoglobin and on the alveolar volume sampled, results are adjusted for these factors (MacIntyre 2005; Graham 2017; West 2012).

Clinical relevance

Diffusing capacity is a reference measurement of gas-transfer efficiency that helps characterize the physiological consequences of parenchymal and pulmonary-vascular disease. A reduced DLCO points toward loss of functioning alveolar-capillary surface or impaired transfer, whereas changes in the transfer coefficient help interpret whether a low value reflects membrane, volume, or blood-related factors. This entry explains what the test measures and how it is interpreted in general terms and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

Measurement is standardized by the joint ATS/ERS statements, originally the 2005 single-breath standardization (MacIntyre 2005) and the 2017 ERS/ATS update (Graham 2017), with interpretation following the 2022 interpretive-strategies standard (Stanojevic 2022).

History

Marie and August Krogh demonstrated diffusion across the lung in the early twentieth century, and the single-breath carbon monoxide method was developed by Marie Krogh and refined through the mid-twentieth century. Roughton and Forster's 1957 analysis separated the membrane and capillary-blood contributions to gas transfer, and ATS/ERS standardization statements (2005, 2017) later made the test reproducible across laboratories.

Debates

How to interpret DLCO relative to alveolar volume

Whether a low DLCO reflects true transfer impairment or simply a reduced alveolar volume is a recurring interpretive question; the transfer coefficient (KCO) helps but does not fully resolve the distinction, and standards caution against over-simplified correction.

Key figures

• Neil MacIntyre
• Brian L. Graham
• Francis J. W. Roughton
• Robert E. Forster

Seminal works

• macintyre-2005
• graham-2017
• west-2012-textbook

Frequently asked questions

Why is carbon monoxide used to measure diffusing capacity?

Carbon monoxide binds hemoglobin so avidly that its capillary partial pressure stays near zero, making its uptake limited by diffusion across the membrane rather than by blood flow - which is exactly what the test aims to measure.

What does a reduced diffusing capacity indicate?

It indicates less efficient gas transfer, which can arise from loss of alveolar-capillary surface area, thickening of the membrane, reduced pulmonary capillary blood volume, or low hemoglobin; the transfer coefficient and clinical context help distinguish these in general terms.