Spirometry is likely mislabeling a significant percentage of smokers as healthy, according to research published in the European Respiratory Journal by Weill Cornell Medicine investigators. Researchers also found that lung diffusion testing (DLCO) can more accurately represent this risk.

To find out whether low-diffusing capacity levels correlated with an increased risk for developing COPD, investigators compared two groups of smokers — one with normal diffusing capacity levels and one with low diffusing capacity levels — all pulled from a larger pool of 1,570 New York City-based smokers with normal spirometry measures.

Once divided into these groups, researchers randomly selected 59 people with normal diffusing capacity and 46 people with low diffusing capacity and followed them for about four years. Throughout this time, investigators ran sporadic lung function tests. All participants continued to smoke throughout the study, and most were about 50 years old, the typical age at which symptoms of lung disease appear.

While the vast majority of participants (97%) with both normal spirometry tests and normal diffusing capacity stayed COPD-free over the four-year term, the researchers found that 22% of people with the low diffusing capacity developed COPD during the study.

“The problem with the spirometry test is that it doesn’t give you the whole picture of lung function,” said Dr Ronald Crystal, chairman of the Department of Genetic Medicine and the Bruce Webster Professor of Internal Medicine at Weill Cornell Medicine. “Which is where the second, more comprehensive capacity diffusing test, which measures the function of a patient’s air sacks, comes into play.

“If you think of the lung like an upside down tree, the trachea, or windpipe, is the trunk, the branches are the airways and the leaves are the air sacks,” Dr. Crystal explained. “With smoking, those air sacks can be destroyed, and the diffusing capacity test measures their function.”

Although it is similarly noninvasive, the diffusing capacity test is less portable, more expensive, and more difficult to run. For this reason, it is typically only used by pulmonologists.

“This shows that smokers with normal spirometry results but low diffusing capacity are at significant risk for developing this serious and fatal disease,” Dr Crystal said.

It also means that many people who have normal spirometry results but never see a specialist or receive the diffusing test are being falsely told not to worry and that they’re okay, Dr. Crystal continued. “Even with a normal spirometry test, patients and their doctors still have to worry that they may be at significant risk for developing COPD.”