More than 2.5 million Canadians have been told they have asthma, but research suggests that as many as 30 per cent of them may have been misdiagnosed.

Ottawa Hospital respirologist Dr. Shawn Aaron, a senior scientist at the Ottawa Hospital Research Institute, published a study in the Canadian Medical Association Journal in 2008, that found many Canadians who believe they have asthma have never been properly tested for the condition.

Aaron, who is now in the process of updating those findings, says he believes the problem of overdiagnosis persists.

“We think it’s because many Canadians are being diagnosed without confirmatory tests being done,” Aaron told CTV’s Canada AM Monday from Ottawa.

In many cases, he says, patients are seeing their family doctors complaining of shortness of breath, wheezing or coughing and they’re given an asthma diagnosis without undergoing any tests.

The proper way to diagnose asthma, he says, is with a spirometer test, which measures lung volume and airway flow.

“All it is is patients blow as hard as they can into a tube and we measure the flow of air that comes out as they blow,” he said.

“It’s very easy to do, it takes about 10 minutes, costs about $15 and it’s completely safe. The only problem is it’s a bit inconvenient, so physicians tend to skip the test.”

The problem with misdiagnosing asthma, says Aaron, is that patients will be prescribed inhalers they cannot benefit from and that won’t treat the real cause of their shortness of breath.

“So you have all the opportunity for side effects with about any opportunity for benefit,” he said.

For those who truly have asthma, the inhaled steroids “work incredibly well,” Aaron says, and can protect patients from the potentially fatal effects of asthma attacks.

“If you have asthma you should be taking these inhalers. They’re going to help you and you’ll put up with the side effects because the disease is much worse than the potential side effects,” he said.

But he says, for those without asthma, inhalers’ side effects are not worth it. Those side effects can include oral thrush, which is a buildup of fungus in the mouth, voice hoarseness, and easy bruising. Long-term use can cause growth suppression in children, cataracts, osteoporosis and glaucoma.

The bottom line, Aaron says, is that patients who suspect they have asthma should insist on proper spirometry testing. And he wants his fellow physicians to take the time to do the proper testing to ensure they get their diagnoses right.

“Why aren’t we doing it? I’m not sure. That’s why I’m here today,” he said.