Asthma may be over-diagnosed by as much as 30 per cent in Canadian adults, according to a new study, which suggests that doctors may be missing the signs of a more serious problem.

"The myth-busting part of this study may be that there are a lot of Canadians who are labeled as asthmatics who believe they have the disease, but they may not have it," Dr. Shawn Aaron, a senior scientist at the Ottawa Health Research Institute and the head of respiratory medicine at the Ottawa Hospital, told CTV News.

Researchers used accepted clinical diagnostic guidelines to re-test nearly 500 Canadians who had been diagnosed with asthma.

They found that 30 per cent of those patients did not appear to have asthma, and two-thirds were able to stop taking asthma medications without experiencing adverse health effects.

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

The research team also included scientists from the Canadian Institutes of Health Research (CIHR). The findings are published in the November 18 edition of the Canadian Medical Association Journal.

Current clinical guidelines recommend that doctors test for asthma by using a spirometer to measure lung volume and airway flow. But many family physicians appear to diagnose asthma based on patients' reported symptoms.

"I don't think it's laziness," Aaron told CTV News. "I think to some extent the tests are not being done because they are not convenient -- they're not located on every street corner."

The Asthma Society of Canada is urging doctors to carry out more spirometer tests.

"We know in studies done by the Asthma Society of Canada that less than 50 per cent of bona fide asthmatics have ever had any type of spirometry," said spokesperson Dr. Mark Greenwald.

The researchers had set out to determine if people who are obese are more likely to be misdiagnosed with asthma.

However, they found that misdiagnosis is just as common among those who are of normal weight.

About five per cent of people in North American have asthma, while about 10 per cent of obese people have the disease.

Because asthma rates have nearly doubled in the last 20 years, "researchers have been examining diagnostic procedures to ensure that asthma is not being over-diagnosed and over-treated," Dr. Peter Liu, scientific director of the CIHR Institute of Circulatory and Respiratory Health, said in a statement.

Aaron also warned patients who have been diagnosed with asthma to not stop taking their prescribed medication until they can consult with a doctor.

With a report by CTV's Rosemary Thompson