TORONTO - Almost a third of adult Canadians diagnosed with asthma and taking medications to treat their wheezing, coughing and shortness of breath may not actually have the allergic respiratory condition at all, researchers say.

In a study of about 500 adults who had previously been told they have asthma, airway-function and other tests showed that about 30 per cent did not in fact have the disease, suggesting that asthma may be significantly overdiagnosed in Canada.

"What we think is happening ... is that if patients come to a physician complaining of shortness of breath or cough or wheeze, oftentimes the physician will assume that the diagnosis is asthma, and based on the patient's history and complaints prescribe medicines to treat asthma," lead investigator Dr. Shawn Aaron said from Ottawa.

"And what they won't do is order the appropriate diagnostic test to confirm asthma."

Aaron, head of respiratory medicine at Ottawa Hospital and the University of Ottawa, said a doctor's suspicion that a patient has asthma should be followed up by confirmatory tests, including spirometry.

A spirometer measures a person's lung volume and airway flow, which can flag whether airways are likely narrowed by inflammation, as is the case with asthma.

When the researchers tested the 496 study subjects with a diagnosis of asthma, they found 150 did not test positive for the condition, Aaron said. "And when we stopped their medicine and assessed them, we weren't able to find asthma."

Yet these patients had been taking asthma medications - typically inhaled steroids, also known as puffers - for an average of 15 years, say the researchers, whose paper is published in this week's issue of the Canadian Medical Association Journal.

"This is a serious issue because asthma medications are expensive and they can have side-effects," Aaron said. "Also, an inappropriate diagnosis of asthma may obscure the true cause of a patient's symptoms."

While inhaled corticosteroids are generally considered safe drugs, higher rates of osteoporosis, glaucoma and cataracts have been seen in patients after long-term use.

But he cautioned that patients shouldn't forgo asthma drugs in the belief they may have been misdiagnosed, because the airway-clogging condition can be life-threatening. Instead, they should ask their doctor for the confirmatory tests.

"I don't want people to assume that they've been overdiagnosed and stop their medicine. That would be very dangerous."

An estimated three million Canadians have asthma, about 12 per cent of them children, says the Asthma Society of Canada on its website. Prevalence of the disease - which kills about 20 children and 500 adults across the country each year - has been on the rise over the last 20 years around the world.

Commenting on the study, Toronto respirologist Dr. Ken Chapman agreed that asthma is overdiagnosed and spirometry tests are not performed as often as they should be.

"They're saying things near and dear to my heart and near and dear to the hearts of all practising lung doctors," Chapman, director of the Asthma Airway Centre at University Health Network, said of the study authors.

"We're endlessly frustrated that doctors think that they can manage lung disease without measuring lung function."