COMMENTARY

How to Interpret a Diffusion Capacity of the Lung for Carbon Monoxide

Aaron B. Holley, MD

Disclosures

March 07, 2017

Background

The diffusion capacity of the lung for carbon monoxide (DLCO) is used to measure the body's ability to transfer oxygen across the alveolar-capillary membrane.[1] Internists use this test to identify interstitial lung disease, oncologists order it to assess the effects of chemotherapy, and pulmonologists use it for all sorts of reasons. DLCO results determine whether a patient can tolerate lung resection or undergo lung volume reduction surgery.[2,3] In summary, it's a pretty big deal.

Unfortunately, interpretation is more difficult than most people realize. For one, results are dependent not just on respiratory function but also on cardiovascular health and hemoglobin (Hb) levels.[4] In addition, the lung volume at which the DLCO is measured significantly influences the result. Relative effort (ie, taking an adequate breath) during the test is critical. Pulmonologists own this test, and my experience is that most ignore its complexity. Volume adjustment is treated as an article of faith (ie, "Do you believe in volume adjustment?") as opposed to a physiologic reality. Hb status is often ignored. I'm willing to bet that internists and oncologists also oversimplify DLCO results.

Evaluating a Noninvasive Test for Hemoglobin

A new study[5] published in the Annals of the American Thoracic Society addressed one of these problems. Noting that it's generally impractical to draw blood from every patient who undergoes DLCO, the authors evaluated use of noninvasive point-of-care (POC) testing for Hb adjustment in real time. They compared Hb assessment via standard blood draw with POC testing using the Pronto-7® (Masimo; Irvine, California) testing device in 205 prospectively enrolled patients. Hb results obtained using each modality were used to calculate an adjusted DLCO, and results were compared.

Their findings were statistically favorable, which is to say that correlation and agreement between each Hb testing modality were good to excellent. DLCO adjustments were very similar. Agreement fell when the 25% of patients with anemia were evaluated, and this is concerning because these are the patients most in need of adjustment. Still, a noninvasive, accurate POC test for Hb could increase confidence in DLCO results.

In summary, the Pronto-7® looks promising, although the manufacturer's website says the device is no longer available. Even if additional studies confirm these findings, physicians would be well advised to understand as much as possible about DLCO interpretation. I'd recommend reading the excellent physiology review published in the American Journal of Respiratory and Critical Care Medicine in 2012.[4]

Unless all facets of the DLCO test—volume, effort, Hb, and cardiac output—are taken into account, inaccurate conclusions will be drawn.

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