In a recent study from Canada, researchers set out to determine how many obese individuals had been misdiagnosed with asthma. What they found was quite interesting. Nearly 30% of all patients in the study, obese or not, had been misdiagnosed. That’s a pretty big number. Can it really be true?
As with most things in medicine, the devil is in the details. Close examination of the study methods reveals some potential biases and limitations that make the true number of asthma misdiagnoses lower. Still, the authors bring to light some important truths about the diagnosis of asthma. First, asthma can be difficult to diagnose. Second, objective measurements of lung function are woefully underutilized and often poorly interpreted. Third, diagnostic challenge testing is also underutilized.
Doctors like to use pithy sayings to guide their practice habits. These are often referred to as clinical pearls. I heard two such sayings often in my training, “If you hear hoof beats, don’t go looking for Zebras.” and “Common things are common.” So when a person comes to me and says, “When I get around my girlfriend’s cat my chest feels tight.”, I’m thinking asthma. The history is often not so straightforward, however. That’s why lung function measurements, spirometry, are so important. If a person is very symptomatic and his spirometry is normal, that makes the diagnosis of asthma less likely. Conversely, if the spirometry looks like asthma and changes a certain amount following treatment with albuterol, that can be diagnostic for asthma. If I see enough red flags in the history and work-up, I’ll often perform a methacholine challenge test. This test gives subjects gradually increasing doses of inhaled methacholine, a chemical which makes asthmatic’s lungs constrict slightly. After each dose, spirometry is performed. If the lung function falls a certain amount, that indicates asthma. The test can take up to a couple of hours.
Back to the study. As I said, the take home message is right on, but I doubt the numbers are very accurate. First, asthma was defined as, “Has a physician ever told you you have asthma.” This is a pretty loose definition. Second, the average time since diagnosis was 15 years. This means that if you are 21 and a doctor told you at age 6 that you have asthma, you could be in the study. Third, only 20% of the misdiagnosed group vs. 45% of the confirmed group were taking daily asthma meds. This tells us that the misdiagnosed group had milder symptoms and likely a less certain clinical diagnosis to begin with. Finally, the phone call method of selection may have led to more patients who already doubted their diagnosis to opt for the study.
If you have questions about asthma, your local allergist is an asthma expert who can answer them.