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Anyone caring for patients in a clinic setting knows that asthma can easily be overdiagnosed. Asthma lacks a gold standard test for diagnosis, can produce vague symptoms, and inexpensive, low risk treatments are available (inhaled corticosteroids and albuterol). The result is many dyspneic or coughing patients are given an asthma diagnosis that is provisional or tentative -- whether or not the patient is told this.
Further, some patients correctly diagnosed with asthma may enter a spontaneous remission, at which time medications should be discontinued. Most patients stop their treatments on their own for cost and convenience reasons, but some may linger on unneeded treatments for years. The rate of spontaneous remission of asthma is unknown.
Just how often asthma is misdiagnosed, over-diagnosed, or over-treated is anyone's guess. A study in JAMA sheds some light.
Authors enrolled 700 Canadian adults who had been diagnosed with asthma by a physician. Patients were randomly selected by telephone outreach (robo-calls).
Subjects were tested with spirometry and most interesting, repeated methacholine challenge tests. Their daily asthma treatments were discontinued (gradually) and they recorded their symptoms and peak flows at home. If symptoms concerning for asthma occurred, patients were seen by a pulmonologist who made the final asthma diagnosis. If no asthma symptoms occurred, patients underwent serial methacholine challenge testing over one year. Almost 90% of patients completed the study.
One Third of "Asthma" Patients Don't Have Asthma?
In fully one-third (203 patients), asthma was ruled out: they had no signs of asthma on testing, and no symptoms during one year of follow-up. After an additional year of follow-up, only 6 of these patients experienced any asthma symptoms. Sixteen more had positive methacholine tests but no symptoms, and after 2 years, 30% of the original cohort remained asthma-free by any definition.
Among the patients determined not to have asthma, less than half had spirometry at the time of their original diagnosis. Only 56% of patients with a confirmed diagnosis of asthma had ever undergone spirometry.
Only about one in 50 patients diagnosed with asthma actually had a missed diagnosis of a more severe condition like congestive heart failure or COPD that was causing their symptoms.
This study replicates almost exactly the 2008 finding that 30% of patients diagnosed with asthma had no symptoms or signs of the condition on repeated testing for 6 months after stopping all treatments.
Asthma Overdiagnosis, Misdiagnosis, and Obesity
As the prevalence of obesity has reached 30% in developed countries, it's likely that both correct and incorrect diagnoses of asthma have risen as well.
People with obesity have an increased likelihood of an asthma diagnosis; whether obesity somehow directly "causes" asthma is a much harder question to answer.
Obese patients have an altered biochemical and clinical phenotype of asthma. Obese asthmatics have a reduced response to inhaled corticosteroid and other treatments. They have worse health-related quality of life with more bad health-related days in general, including from respiratory symptoms. They are 4 times more likely to be hospitalized for asthma. The American Thoracic Society has even proposed a new asthma subtype, "obesity-associated asthma," to reflect these particularities.
It's also easy for obese patients to be misdiagnosed with asthma. Obesity causes dyspnea on exertion (although not bronchial hyper-responsiveness) and impairs respiratory muscle function. Obese people are more likely to report wheezing in general than non-obese people. A majority of patients with severe or hard-to-control asthma are obese, but with no gold standard test for asthma, it's impossible to know how factors other than asthma may exaggerate their asthma's apparent severity.
In the aforementioned 2008 study, significantly more obese patients had their asthma excluded after careful evaluation, compared to non-obese people. Authors in the 2017 JAMA study did not say whether BMI influenced asthma remission or misdiagnosis.
Medical practice doesn't allow for probabilistic diagnoses; physicians are forced to commit to their best guess of a diagnosis, even when they doubt it. Because of the nature of asthma, and the people who present with respiratory symptoms, it makes sense that diagnosis will often be wrong.
Reevaluation of Diagnosis in Adults With Physician-Diagnosed Asthma. JAMA. 2017 Jan 17;317(3):269-279.