Asthma may be overdiagnosed in countries like Canada, suggests a longitudinal study of 540 obese and non-obese adults ( found approximately one third of Canadians with physician-diagnosed asthma do not have asthma when objectively tested.

Asthma rates have increased in Canada and the US by 75% between 1980 and 1994, and studies suggest a possible link between obesity and asthma. In North America, obese adults are twice as likely to be diagnosed with asthma by a physician as non-obese adults. However, the "study found that 30% of adults recruited from the community who had been diagnosed with asthma by a physician had no evidence of asthma when their medications were tapered and when there were evaluated with serial assessments of symptoms, lung function and bronchial provocation tests," state Dr. Shawn Aaron from the Ottawa Health Research Institute and coauthors. "Overdiagnosis, or misdiagnosis, of asthma in Canada seems to be very common, but obese adults are not more likely to be overdiagnosed compared to normal weight adults." 

Potential participants for the study were recruited by random telephone digit dialling from 8 Canadian cities and nearby rural areas from British Columbia to Nova Scotia, comprising a sample representative of the national population.

The researchers write that this overdiagnosis of asthma is concerning and suggest that physicians use spirometry, which measures the rate and volume of airflow through the lungs, for objective testing before labelling respiratory symptoms. 

A related editorial by Dr. Matthew Stanbrook and Dr. Alan Kaplan and the CMAJ editorial team ( caution that asthma may be misdiagnosed rather than overdiagnosed. Symptoms attributed to asthma may signify other underlying medical conditions. They agree that all suspected cases of asthma must be diagnosed with spirometry and this should be available in all practice settings.