Led by Ottawa researcher Dr. Shawn Aaron, the study examined 496 people from eight Canadian cities who reported receiving a diagnosis of asthma from a physician.
When the participants were retested for asthma using the accepted clinical guidelines, it was found that 30 per cent had no evidence of asthma. Two thirds of these individuals were able to safely stop taking asthma medications.
"Our study suggests that there may be a substantial over-diagnosis of asthma in Canadian adults," said lead author Dr. Shawn Aaron, a Senior Scientist at the Ottawa Health Research Institute and Head of Respiratory Medicine at The Ottawa Hospital and the University of Ottawa.
"This is a serious issue because asthma medications are expensive and they can have side effects. Also, an inappropriate diagnosis of asthma may obscure the true cause of a patient's symptoms," Aaron said.
The study's original objective was to determine if obese people were more likely to be misdiagnosed with asthma, but the results showed that misdiagnosis was just as common in people of normal weight.
The prevalence of asthma in Canada and America is five per cent overall, and 10 per cent for obese people. The overall prevalence has nearly doubled in the last 20 years.
Previous studies have suggested that asthma is often diagnosed by family physicians solely on the basis of symptoms, despite the fact that clinical guidelines recommend using a spirometer to objectively measure lung volume and airway flow.
A spirometer costs a few thousand dollars and can be set up fairly easily in any clinic.
"The message for patients is that if you've been diagnosed with asthma and you have not had a spirometry test, you should ask your physician for one. The other important message is that asthma can be deadly, so you should never stop taking a medication without consulting a physician." said Aaron.
The study is published in the November 18, 2008 edition of the Canadian Medical Association Journal.