Chart — Pulmonary Function Testing
DLCO Interpretation
DLCO is what turns a spirometry pattern into a named diagnosis. Pair the obstructive or restrictive pattern with a low, normal, or high diffusing capacity and the differential narrows quickly. Always correct DLCO for hemoglobin first, since the blood count moves it independent of the lungs.
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.
Pattern + DLCO
| Spirometry Pattern | DLCO | Points To | Why |
|---|---|---|---|
| Obstruction | Low | Emphysema | Loss of alveolar-capillary surface area. |
| Obstruction | Normal | Asthma or chronic bronchitis | Airways narrowed, membrane intact. |
| Restriction | Low | Interstitial lung disease / pulmonary fibrosis | Thickened membrane and lost units (parenchymal). |
| Restriction | Normal or high | Chest-wall or neuromuscular restriction | The lungs are squeezed but the parenchyma is intact. |
| Normal spirometry | Low (isolated) | Pulmonary vascular disease, early ILD, or anemia | Reduced capillary blood volume or hemoglobin, or early parenchymal change. |
| Any pattern | High | Alveolar hemorrhage, polycythemia, left-to-right shunt | Extra hemoglobin or blood available to take up CO. |
How to Use This Chart
- DLCO turns a pattern into a diagnosis: obstruction with a low DLCO is emphysema, but obstruction with a normal DLCO is asthma or chronic bronchitis.
- Restriction with a low DLCO is parenchymal (ILD); restriction with a normal or high DLCO is extra-parenchymal (chest wall or neuromuscular).
- Always correct DLCO for hemoglobin first — anemia lowers it and polycythemia raises it independent of the lungs.
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.