As many as one in three adults diagnosed with asthma may not actually have the chronic lung disorder, a Canadian study suggests.

Researchers did lung function tests on 613 adults who had been diagnosed with asthma within the past five years. If participants took asthma medicines, researchers gradually weaned them off the drugs over four clinic visits to see how well their lungs worked without treatment.

The evaluations ruled out asthma in 203 of the participants, or 33 percent. After one year of follow-up, 181 of these people still did too well on lung tests to be diagnosed with asthma, researchers report in JAMA.

“We were able to get these patients completely off asthma medications, and they did well in follow up over the next year despite remaining off medications,” said lead study author Dr. Shawn Aaron of the University of Ottawa and the Ottawa Hospital Research Institute.

“Some of these patients were clearly misdiagnosed to begin with, and they had other conditions other than asthma, and some did have asthma but it was in remission,” Aaron added by email.

Asthma can be difficult to diagnose because not all patients have the same triggers or symptoms, which can include difficulty breathing, chest pain, cough and wheezing. Some chronic asthma patients experience periods of remission and relapse.

For the current study, researchers had all of the patients monitor symptoms and do breathing tests at home to see how fast air comes out of their lungs, a measurement known as peak expiratory flow.

All of the participants also did bronchial challenge tests. For these assessments, patients inhaled a medication that causes the bronchial tubes to constrict, simulating conditions that can cause asthma to see how well airways react.

Each patient also did spirometry tests that measure lung function by seeing how much air people inhale, how much they exhale and how fast they exhale.

Participants in whom a diagnosis of current asthma was ultimately ruled out were followed up clinically with repeated bronchial challenge tests over one year.

Among those misdiagnosed with asthma, 12 people, or 2 percent of the participants, had serious conditions other than asthma, like heart disease and pulmonary hypertension, the study found. Still others were found to have conditions like chronic obstructive pulmonary disease (COPD), gastroesophageal reflux (GERD) or anxiety-related hyperventilation rather than asthma.

Those who were misdiagnosed were less likely to have had airflow limitation tests when they were originally diagnosed, compared with participants who had their original asthma diagnosis confirmed in the current study.

For patients who had asthma ruled out, 90 percent had asthma medications safely stopped for one year after being weaned off drugs for the study.

One limitation of the study is that researchers only followed patients for a total of 15 months, which isn’t long enough to rule out the possibility that some patients in remission might have asthma symptoms in the future, the authors note. The study also excluded patients using long-term oral corticosteroids, leaving only people with milder forms of asthma to participated.

Still, the study reaffirms the need for patients who have been diagnosed with asthma to have their diagnosis confirmed with objective lung function testing, particularly spirometry, before being started on lifelong therapy, Dr. Alan Kaplan, a researcher at the University of Toronto who wasn’t involved in the study, said by email.

“The most important potential harm of misdiagnosis of asthma is not treating the patient’s actual disease,” said Dr. Helen Hollingsworth of Boston University, co-author of an accompanying editorial.

“For other patients, not recognizing that asthma is in remission, can lead to taking unnecessary medication,” Hollingsworth added by email. “While the adverse effects of asthma medication are minimal, no one wants to take unnecessary medication.”