DLCO
DLCO measures not how much air you move, but how well gas crosses from the air sacs into the blood. You inhale a single breath containing a trace of carbon monoxide, hold it about ten seconds, then breathe out; the amount of carbon monoxide that disappeared into the blood tells how good the gas-transfer surface is.
Carbon monoxide is used because it binds avidly to haemoglobin, so its uptake is limited almost entirely by the lung's diffusing surface rather than by blood flow. A low DLCO means the alveolar–capillary membrane is damaged, thinned out, or simply has too little area or too little blood to transfer gas efficiently.
DLCO is one of the most informative lung tests because it integrates the whole gas-exchange machinery. It falls in emphysema (lost alveoli), in interstitial fibrosis (thickened membrane), and in pulmonary vascular disease (lost capillaries). It can rise transiently when blood floods the alveoli, as in alveolar haemorrhage.
Two patients have the same spirometry, but one has a normal DLCO (chronic bronchitis) and the other a DLCO of 40 percent predicted (emphysema) — the diffusing capacity separates the two faces of COPD.
DLCO probes the quality of the gas-exchange surface itself.
DLCO depends on haemoglobin level, so the result is usually corrected for anaemia; without correction, anaemia falsely lowers it and polycythaemia falsely raises it.