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One-third of Canadian adults who have been told they have asthma may have been wrongly diagnosed by their doctor, according to a new study that reveals serious problems with the way the disease is identified and treated in Canada.

The findings, being published Tuesday in the Canadian Medical Association Journal, suggest thousands may needlessly be taking medications that could have life-altering side effects and major costs.

“There's a lot of drugs being consumed and paid for that really are not likely to have any therapeutic role for these patients,” said Shawn Aaron, head of the respirology division at the University of Ottawa and lead author of the study. “If this study is true, and I have no reason to believe it's not, there are 30 per cent of people walking around in Canada who believe they have asthma, whose physicians have told them they have asthma, who don't [have asthma].”

About 8.3 per cent of Canadians aged 12 and older were identified as having asthma in 2005, according to Statistics Canada. In 2007, 3.4 million prescriptions were issued for the top five asthma medications, at a cost of nearly $329-million, according to IMS Health Canada, a company that tracks the pharmaceutical industry.

The reason for the inflated rates of asthma diagnosis seems to be a lack of adequate testing, the study says.

It is widely estimated that less than half of Canadians diagnosed with asthma undergo spirometry, a relatively simple test used to measure lung function. It is considered one of the most efficient ways to diagnose asthma, but it is costly and time consuming, which could be a problem in some areas. But doctors should refer patients to community laboratories for the test, Dr. Aaron said.

“[A] doctor would never diagnose high blood pressure without actually taking a blood-pressure reading,” Dr. Aaron said. “Physicians in Canada and all over the world are not objectively confirming the diagnosis of asthma.”

Some members of asthma organizations yesterday questioned the findings.

“I doubt the number [of misdiagnosed asthma cases] is that high if we try to apply this to the population of Canadians at large,” said Mark Greenwald, chair of the science and medicine committee at the Asthma Society of Canada. “We still have the majority of our patients properly diagnosed.”

But Dr. Aaron said that study participants went through a multitude of tests over a two-month period.

“I think if we decide not to believe this study, that's anybody's prerogative, but I think they'd be putting their head in the sand and pretending there isn't a problem when there is one.”

Study participant Sue Abou-Hamda, 41, has first-hand experience with an asthma misdiagnosis. For years, the Ottawa resident had allergies and occasional wheezing. When she was in the hospital giving birth to her first child, she was told she had asthma. She eventually had a spirometry test, and while she was told she “passed,” her doctor insisted she had mild asthma and prescribed medication.

“I was told I had asthma and that I needed to take inhalers for the rest of my life, otherwise risk terrible health problems later,” Ms. Abou-Hamda said.

She said her life was restricted in many ways. She stayed indoors during smog alerts and gave away her two long-time pets on the advice of her doctor when she became pregnant a second time.

When the study's researchers told her she didn't have asthma, Ms. Abou-Hamda was relieved but distraught.

“It is very upsetting,” she said. “I had been taking inhalers every day.”

One of the greatest concerns about asthma over-diagnosis is that many people may be unnecessarily taking powerful drugs that could affect their health.

Some of the inhaled steroids used to control the symptoms of asthma have been linked to osteoporosis, cataracts and glaucoma, Dr. Aaron said.

Health Canada has issued warnings in recent years about the possible increased risk of death linked to some asthma medications, including Advair and Symbicort.

“If you're taking a medicine you don't need, obviously you're exposing yourself to risk,” Dr. Aaron said.

Nearly 66 per cent of those involved in the study who found out they didn't have asthma took no asthma drugs during the six-month follow-up period. Others took them for a short time or saw their doctor for respiratory symptoms.

The study's original purpose was to determine whether obese people are misdiagnosed with asthma more often than non-obese patients.

Researchers examined two groups of adults who had been diagnosed with asthma, 242 of them obese, and 254 not obese.

Researchers found that misdiagnosis of asthma was just as likely among the obese as the non-obese. That surprised researchers, who expected to find more over-diagnosis among the obese. Obesity has been linked to a significantly increased risk for asthma and researchers hypothesized the increased risk could be explained by misdiagnosis.

Public education and promotional campaigns by non-profit groups and drug companies could be helping to drive up diagnosis rates by predisposing physicians and patients to the symptoms associated with asthma, according to the study.

Dr. Aaron said people who doubt their diagnosis should stay on their medication until they can see their doctor and have a detailed evaluation.

The study was funded by the Canadian Institutes of Health Research and the division of respiratory medicine at the Ottawa Hospital.