reduced DLCO

DLCO measures how efficiently gas crosses from the air sacs into the blood. The test uses a tiny, harmless amount of carbon monoxide as a stand-in for oxygen, because it is taken up avidly by haemoglobin. A reduced DLCO simply means that this transfer is impaired: gas is not moving from lung to blood as well as it should.

The transfer can falter in several ways. The membrane between air and blood may be thickened or scarred, as in interstitial lung disease; the surface area for exchange may be lost, as in emphysema; or the blood available to pick up gas may be reduced, as in pulmonary vascular disease or anaemia. Because of this, a low DLCO is sensitive but not specific.

In interstitial and vascular lung diseases, a falling DLCO is often one of the earliest and most useful signals, sometimes dropping before lung volumes change much. Tracking it over time helps gauge whether a disease is progressing and how a person might tolerate exertion.

Interpreting the result requires context. The value is usually corrected for the patient's haemoglobin, and it must be read alongside lung volumes and the clinical picture, since the same low number can point to very different problems.