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MONDAY, Nov. 17 -- Almost one in three adults who've been told they have asthma may not have the chronic airway disease, new Canadian research claims.

The study, published in the Nov. 18 issue of the Canadian Medical Association Journal, included nearly 500 adults from eight Canadian cities. Researchers found that about 30 percent of those diagnosed with asthma had been misdiagnosed.

"When we evaluated these people with lung function tests and then took them off their medications, we couldn't find asthma in 29 percent of the non-obese and 32 percent of the obese," noted study author Dr. Shawn Aaron, head of the division of respiratory medicine at the Ottawa Health Research Institute at the University of Ottawa, Ontario.

Aaron said the original goal of the study was to see if asthma was misdiagnosed more often in obese individuals, because the prevalence of asthma is twice as high in this group. However, he said, there was no statistical difference in the rates of overdiagnosis based on body weight.

And even though the study was not designed to confirm or refute the idea that the obese are more likely to have asthma, it did support the notion that obese people truly do have a higher incidence of the disease.

For the study, the researchers randomly recruited 496 Canadian adults -- 242 obese and 254 non-obese -- from eight Canadian cities. All had been diagnosed as having asthma by physicians.

The study participants underwent lung function testing, and if they performed well on the lung function test, were given an airway "challenge" test. People with asthma aren't expected to do well on this test.

Those that passed the airway challenge were gradually tapered off their asthma medications. Aaron was quick to point out that this is not something anyone should attempt on their own; it should only be done under a doctor's supervision.

Based on these tests, the diagnosis of asthma was excluded in 28.7 percent of the non-obese and 31.8 percent of the obese.

After a six-month follow-up period, Aaron said that only 8 percent had symptoms significant enough to warrant a health-care visit.

Aaron said that some of the misdiagnosed cases could result from bad viral infections, the symptoms of which can last for several months. And the main reason people are misdiagnosed is because doctors don't always perform a key lung function test called spirometry.

In the same issue of the journal, editorial authors Drs. Matthew Stanbrook and Alan Kaplan pointed out that trying to manage asthma without doing lung function testing was akin to trying to manage high blood pressure without taking blood pressure readings or high cholesterol without measuring cholesterol levels in the blood.

"You need to have some objective measure. You can't diagnose asthma based on patient symptoms alone," added Dr. Jennifer Appleyard, chief of allergy and immunology at St. John Hospital in Detroit. She said the issue of misdiagnosis is of concern in the United States, just as it is in Canada.

Appleyard agreed that viral infections can be mistaken for asthma, but said that other serious problems can also be mistaken for asthma, such as chronic obstructive pulmonary disease (COPD) and congestive heart failure.

Both Aaron and Appleyard said they hope these findings don't cause doctors to stop diagnosing asthma, because Appleyard said asthma is still underdiagnosed in some cases. But they suggest that more doctors should include spirometry in their practice or refer their patients to doctors who do perform spirometry to confirm the asthma diagnosis.

More information

To learn more about asthma, visit the Nemours Foundation - http://kidshealth.org/parent/asthma_basics/

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