Overdiagnosis of asthma in the community.

BACKGROUND: Overdiagnosis of asthma may be an emerging problem after years of attention to the rising prevalence and reported underdiagnosis of the disease.
OBJECTIVES: A sample of adult asthmatics from the community was investigated to determine whether they met the current diagnostic criteria for asthma.
METHODS: Ninety participants were studied from a self-referred sample of physician-labelled, adult asthmatics from the community. The setting was a tertiary care, university-affiliated teaching hospital in Halifax, Nova Scotia. Three diagnostic criteria from the Canadian Thoracic Society Asthma Guidelines were used to demonstrate the presence of asthma: first, positive symptom history, and either, second, reversible airflow obstruction demonstrable on spirometry or documented peak expiratory flow rate diurnal variability, or, third, bronchial hyperreactivity to methacholine.
RESULTS: At the time of the study, 41% of a sample of physician-labelled asthmatics showed no evidence of reversible airflow obstruction and had a negative methacholine challenge. By backward logistical regression analysis, a higher mean number of medications used (P<0.01), a lower forced expiratory volume in 1 s (P<0.05) and using inhaled steroids (P<0.05) were predictive of meeting the diagnostic criteria for asthma. Sixty-two per cent of subjects who did not meet the criteria for asthma were currently taking medications for "asthma". Only 52.2% of the subjects reported ever having undergone pulmonary function testing.
CONCLUSIONS: Overdiagnosis of asthma is a potential problem, which may result in unnecessary or inappropriate medication use, increased health care costs and mislabelling of patients. The authors recommend greater use of objective diagnostic tests such as spirometry, peak flow diaries and bronchial provocation to establish a clinical diagnosis of asthma.