DLCO and TLCO: Terminology and Definition
DLCO (diffusing capacity of the lung for carbon monoxide) and TLCO (transfer factor for carbon monoxide) are exactly the same test—they are simply different names for the identical measurement, with DLCO being the preferred American terminology and TLCO being the European equivalent. 1
What the Test Measures
DLCO/TLCO measures the uptake of carbon monoxide from the alveolar space into pulmonary capillary blood, following the same pathway as oxygen during normal gas exchange 2
The test provides an integrated picture of gas exchange efficiency by examining how effectively oxygen transfers from alveoli into the blood 3
It is considered the second most important pulmonary function test after spirometry for evaluating lung disease 4
How the Test is Performed
The patient breathes quietly, inhales a gas mixture containing a small amount of carbon monoxide plus a tracer gas (helium or methane), holds their breath for ten seconds, then exhales 5, 4
The test requires very little patient effort compared to spirometry, making it easier to perform in many clinical situations 4
Both the diffusing capacity and alveolar volume (VA) are measured simultaneously, allowing calculation of the transfer coefficient (KCO, also written as DLCO/VA) 5
Terminology Clarification
The official abbreviation is DL,CO (with subscript L and CO), though commonly written as DLCO in clinical practice 1
The term "transfer factor" is synonymous with diffusing capacity—European guidelines use "transfer factor" while American guidelines use "diffusing capacity" for the identical measurement 1
The ratio DLCO/VA is also called KCO (transfer coefficient), though the term "DL/VA" should be avoided because it is frequently misunderstood 5
Clinical Significance
DLCO testing is critical for diagnosing interstitial lung disease, pulmonary vascular disease, and emphysema, and for differentiating causes of dyspnea when spirometry is normal or inconclusive 6, 7
More than 40% of patients with normal FEV1 may have reduced DLCO, making this test essential when symptoms persist despite reassuring spirometry 6
A DLCO below 40% predicted is associated with increased morbidity and mortality, providing important prognostic information beyond diagnosis 4