Guide — Pulmonary Function Testing
Diffusing Capacity (DLCO)
How well gas crosses the alveolar-capillary membrane — what the single-breath DLCO measures, why carbon monoxide is used, the diseases that lower it (emphysema, interstitial disease, pulmonary vascular disease, anemia) and raise it, and the hemoglobin correction.
9 min read · Pulmonary Function Testing
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
The diffusing capacity of the lung for carbon monoxide (DLCO, also called transfer factor or TLCO) measures how readily gas crosses the alveolar-capillary membrane into the blood. It reflects the surface area available, the membrane thickness, the capillary blood volume, and the amount of hemoglobin present to bind the gas. That makes a single number a window into both the lung architecture and the blood carrying gas away from it.
| Determinant | Role | Effect |
|---|---|---|
| Surface area | Area of membrane available for gas exchange | Lost in emphysema → ↓ DLCO |
| Membrane thickness | Distance gas must diffuse across the barrier | Thickened in fibrosis → ↓ DLCO |
| Capillary blood volume | Blood available to carry the gas away | Reduced in PE / pulmonary HTN → ↓ DLCO |
| Hemoglobin | Binding sites for the carbon monoxide | Low in anemia → ↓ DLCO (correct for Hb) |
Key Concepts
- Why carbon monoxide. CO binds hemoglobin so avidly that its uptake is limited by diffusion across the membrane rather than by blood flow, which makes a trace dose an ideal tracer for membrane transfer. The single-breath technique has the patient inhale a trace CO mixture (with a tracer gas) to TLC, breath-hold about 10 seconds, then exhale; the CO taken up gives the DLCO.
- What it depends on. DLCO depends on the alveolar-capillary surface area, the membrane thickness, the capillary blood volume, and the hemoglobin concentration.
- What lowers DLCO.Emphysema (loss of surface area); interstitial lung disease / fibrosis (a thickened membrane and lost units); pulmonary vascular disease, PE, and pulmonary hypertension (reduced capillary blood volume); and anemia (less hemoglobin — so DLCO must be corrected for hemoglobin).
- What raises DLCO. Alveolar hemorrhage (extra hemoglobin in the alveoli binds CO), polycythemia (more hemoglobin), left-to-right shunts and increased pulmonary blood flow, and asthma.
- DLCO/VA (KCO). The transfer coefficient corrects DLCO for the alveolar volume actually ventilated, helping separate a loss of lung units from a true membrane diffusion problem.
Assessment & Findings
DLCO is most powerful read alongside the spirometry pattern — the pattern plus the DLCO narrows the diagnosis.
| Pattern + DLCO | Points To | Note |
|---|---|---|
| Obstruction + low DLCO | Emphysema | Lost alveolar-capillary surface area |
| Obstruction + normal DLCO | Asthma or chronic bronchitis | Membrane intact |
| Restriction + low DLCO | Interstitial (parenchymal) lung disease | Thickened membrane, lost units |
| Restriction + normal/high DLCO | Chest-wall or neuromuscular restriction | Lungs are intact |
| Isolated low DLCO, normal spirometry | Early interstitial or pulmonary vascular disease | Consider PE or pulmonary hypertension |
RT Priorities & Interventions
- Ensure a valid maneuver.An adequate inspired volume (at least 85–90% of the largest VC), a stable breath-hold of about 10 seconds, no leak, and the patient not having smoked or exercised recently (carboxyhemoglobin lowers the measured DLCO).
- Correct for hemoglobin. Always correct DLCO for hemoglobin (and for carboxyhemoglobin) before interpreting it.
- Read it in context. Combine the DLCO with the spirometry pattern to point to the disease.
Common Pitfalls
- Forgetting the hemoglobin correction — anemia falsely lowers DLCO and polycythemia falsely raises it.
- Recent smoking (a high carboxyhemoglobin) artificially lowers the measured DLCO.
- Reading the DLCO in isolation rather than alongside the obstructive or restrictive pattern.
Board Exam Pearls
- DLCO uses carbon monoxide because its uptake is diffusion-limited.
- Low DLCO: emphysema, interstitial fibrosis, pulmonary vascular disease and PE, and anemia.
- High DLCO: alveolar hemorrhage, polycythemia, left-to-right shunt, and asthma.
- Correct DLCO for hemoglobin.
- Obstruction plus a low DLCO is emphysema; restriction plus a low DLCO is interstitial disease.
FAQ
What does DLCO actually measure?
How easily gas crosses the alveolar-capillary membrane into the blood. It depends on the surface area available, the membrane thickness, the capillary blood volume, and the amount of hemoglobin - so it reflects both the lung architecture and the blood.
Why is carbon monoxide used?
Carbon monoxide binds hemoglobin so strongly that its uptake from the alveolus is limited by diffusion across the membrane rather than by blood flow, which makes a trace dose an ideal, sensitive measure of membrane transfer.
What lowers and raises DLCO?
It falls in emphysema (lost surface area), interstitial fibrosis (a thickened membrane), pulmonary vascular disease and PE (less capillary blood), and anemia (less hemoglobin). It rises with alveolar hemorrhage, polycythemia, left-to-right shunts, and asthma.
Why correct DLCO for hemoglobin?
DLCO depends on how much hemoglobin is present to bind the carbon monoxide, so anemia falsely lowers it and polycythemia falsely raises it. Correcting for hemoglobin (and for carboxyhemoglobin from recent smoking) keeps the result a measure of the membrane, not the blood count.
Put it to work
DLCO and the A-a gradient both probe how gas crosses into the blood. Work the alveolar gas math behind oxygenation.
Open the A-a Gradient calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Diffusing capacity chapters.
- Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J. 2017;49(1):1600016.