It is no secret that smoking causes damage to nearly every organ in the human body. But when researchers from National Jewish Health, a hospital in Denver, Colorado took a closer look at long-term smokers, they found most had passed a test that failed to detect signs of lung disease. The study that resulted from the investigation, published in JAMA Internal Medicine, reveals a flaw in widely used lung-function tests that may be leaving millions undiagnosed.

"The impact of chronic smoking on the lungs and the individual is substantially underestimated when using lung-function tests alone," the study’s senior author James D. Crapo, a professor at National Jewish Health, said in a press release. "Lung disease is common in smokers whose lung-function tests fall within population norms."

The research team examined 8,872 people between the ages of 45 to 80, who smoked a pack a day for 10 to 50 years. Half of the group passed a lung-function test, which measures how much air you can take into and blow out of your lungs, and how fast you can do it by blowing into a spirometer. According to the National Heart, Lung, and Blood Institute, the test is designed to diagnose asthma, scarring of the lung, and chronic obstructive pulmonary disease (COPD).

When the group that passed the lung-function test was reevaluated based on CT scans, patients’ physical abilities, use of respiratory medication, and respiratory symptoms, 55 percent actually had respiratory problems. Researchers believe it’s likely that many of those patients have early-stage COPD—an incurable and progressive disease of the lungs. The disease causes inflammation and thickening of the airways, making it the third leading cause of death in the United States.

Early detection of COPD leads to an earlier treatment intervention, ultimately lessening symptoms, slowing the progression, and improving overall quality of life. By demonstrating the lung-function test’s failure, the researchers hope to prove screening with CT scans should be routine in order to detect early stages of lung cancer and COPD.

On the CT scans alone, researchers were able to see airway thickening or emphysema in 42 percent of the undiagnosed participants. Twenty-three percent had a significant shortness of breath, and 15 percent not could walk 350 meters in six minutes, compared to the four percent of those who never smoked. Undiagnosed participants also had a considerably worse quality of life than those who had never smoked.

"Smokers who have 'normal' lung-function tests often have significant respiratory disease," the study’s lead author Dr. Elizabeth Regan, a professor at National Jewish Health, said in a press release. "Many of those smokers likely have the early stages of chronic obstructive pulmonary disease. We hope these findings will help debunk the myth of the healthy smoker and highlight the importance of smoking prevention and cessation to prevent lung disease and other long-term effects of smoking."

Source: Regan E and Crapo JD. JAMA Internal Medicine. 2015.