DLCO Interpretation: How to Read DLCO, VA, and KCO
A guide to reading gas-transfer results on an adult pulmonary function test. Start with the lab’s reference limits and quality notes, then use VA and KCO to understand the pattern.
Four checks before interpreting DLCO
Find the DLCO result and its reference limit
DLCO (also called TLCO) measures carbon monoxide transfer from alveolar gas to blood. Compare the measured value with the lab’s lower limit of normal (LLN) or the corresponding z-score, typically −1.645. Percent predicted alone can miss differences in the normal range across patients.
Read the quality comments
Look for acceptable, repeatable maneuvers and check comments about incomplete inspiration, leaks, or inconsistent efforts. A result near the LLN carries less interpretive weight when the maneuvers are suboptimal.
Check hemoglobin adjustment
Look for the hemoglobin value used and whether the lab accounted for it. Anemia can lower measured DLCO. If adjustment status is unclear, verify it with the lab before attributing a low result to a lung or vascular process.
Read VA and KCO alongside lung volumes
VA is the volume reached by the test gas during the single breath. KCO (sometimes labeled DLCO/VA) is DLCO divided by VA. If total lung capacity (TLC) is available, compare it with VA. A disproportionately low VA can reflect uneven gas distribution or incomplete inspiration.
Common DLCO patterns
KCO changes with VA. When VA is low, a KCO within the lab’s reference range does not correct or cancel out a low DLCO.
| Report pattern | What to consider |
|---|---|
| Low DLCO with VA in range | Consider anemia, pulmonary vascular disease, or emphysema with preserved lung volume. Review hemoglobin, spirometry, and the clinical context even if spirometry is within range. |
| Low DLCO with low VA and low or in-range KCO | This can occur with loss of functioning alveolar–capillary units. Check TLC, the VA-to-TLC relationship, and maneuver quality before attributing the pattern to parenchymal disease. |
| Low DLCO with low VA and high KCO | Check for incomplete lung expansion or localized loss of lung volume, such as after resection. Use the quality notes, TLC, and history to distinguish these possibilities. |
| DLCO near the lower limit | Interpret the value in context. Confirm the reference limit, test quality, and hemoglobin adjustment. If comparing serial reports, verify that the testing and reporting conditions are comparable. |
Three sample reports
These fictional examples use mL/min/mmHg for DLCO and mL/min/mmHg/L for KCO. Each comparison uses the reference limits of that example lab.
Low DLCO with normal spirometry
- DLCO
- 13.8 · below LLN of 18.0
- VA
- 5.0 L · within range
- KCO
- 2.76 · below range
Lab notes · Spirometry and TLC within range. Repeatable maneuvers. Hemoglobin accounted for.
How to read it
The report supports reduced gas transfer despite spirometry within range. The clinical context and other testing help distinguish among possible causes, including a pulmonary vascular process or emphysema.
Low VA with preserved KCO
- DLCO
- 11.4 · below LLN of 17.0
- VA
- 3.0 L · below range
- KCO
- 3.80 · within range
Lab notes · Repeatable maneuvers. Hemoglobin accounted for. TLC unavailable.
How to read it
Gas transfer is reduced. The in-range KCO does not establish that gas transfer is preserved or explain the low VA. Review lung volumes and the inspiratory portion of the maneuver to help distinguish reduced lung volume from uneven gas distribution.
A result just below the lower limit
- DLCO
- 17.2 · below LLN of 17.5
- VA
- 4.8 L · within range
- KCO
- 3.58 · within range
Lab notes · Maneuvers did not meet repeatability criteria. Hemoglobin adjustment status unclear.
How to read it
The reported value falls just below the lab’s limit. Limited repeatability and unclear hemoglobin adjustment reduce confidence in the classification. Clarify those details with the lab before drawing conclusions or comparing with prior tests.
References
- ERS/ATS interpretive strategies for routine lung function tests
Stanojevic et al., 2022. Reference limits, gas-transfer interpretation, and the VA and KCO framework.
- ERS/ATS single-breath DLCO measurement standard
Graham et al., 2017. Maneuver quality, repeatability, and hemoglobin adjustment.