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COPD and asthma are both conditions that cause obstruction of air flow through the lungs. The main clinical difference between COPD and asthma is that the airway obstruction is fully or largely reversible with asthma but not with COPD. Spirometry is the most reliable way to determine reversibility. But there are some other clues that help doctors differentiate the two conditions.
Spirometry allows measurement of the forced expiratory volume in 1 second (FEV1) before and after treatment. An increase or improvement in the FEV1 after treatment reflects the degree to which obstruction is reversible. In other words, it reflects the extent of restoration of normal airflow.
In the case of asthma the use of an inhaled bronchodilator will usually fully or largely relieve airway obstruction. The relief might occur within minutes to hours depending on the severity of the asthma. But in some cases significant reversal of obstruction may not occur for days. Depending on the severity of the disease, asthma attacks might require other medications including inhaled and oral glucocorticoids (medications of the cortisol class) to achieve reversal of airway obstruction.
In contrast to asthma, reversibility of obstruction measured by spirometry is nonexistent or partial in COPD. Spirometry confirms the diagnosis of COPD though only if the FEV1/FVC ratio is less than 70% of predicted. That means the prolonged cigarette smoking has caused airway blockage of such a degree that the resultant reduction in FEV1 is disproportionately greater than that of FVC.
Spirometry and absolute measurements alone are not always reliable for differentiating between asthma and COPD though. Such is the case for a number of reasons.
- First of all, lung function declines with age. Therefore, the FEV1/FVC can be less than 70% of predicted in very elderly individuals, even if they don’t have COPD. Thus, determinations based on whether the readings are above or below the predicted lower limits of normal are more reliable. The pulmonary function test machine that records the readings during spirometry also predicts what the normal FEV1 and FVC values should be. It does so by calculating them using a formula taking the age, height and race of the person undergoing the test into account.
- Secondly, patients with mixed COPD and restrictive lung disease might have an FEV1/FVC ratio well above the lower limit of predicted because of a reduction in their FVC due to the restrictive disease.
- Thirdly, patients with coexisting asthma and COPD might show a degree of reversibility well less than that characteristically seen in asthma alone.
- Fourthly, individuals with severe asthma might have a baseline FEV1 of less than 70% of predicted without significant reversibility in between attacks.
Perhaps the biggest challenge most physicians face is not determining the difference between asthma and COPD in a given patient, but in deciding if both diseases are present. In such cases there is usually an overlap of clinical findings and observations. Therefore, signs and symptoms might support both diagnoses. International guidelines define this state as the asthma-COPD overlap syndrome.
In the case of COPD a long history of cigarette smoking, a barrel-shaped chest oftentimes seen with emphysema, or signs and symptoms of a pink puffer or blue bloater help confirm the diagnosis. The absence of a history of cigarette smoking does not belie the diagnosis of COPD though because of the possibility of alpha 1 antitrypsin deficiency and other less common nonsmoking related causes of COPD.
Confirming asthma when it coexists with COPD is less problematic if there are clues of allergic asthma. Individuals with allergic asthma will oftentimes have a personal and/or family history of other allergic diseases such as hay fever, eczema, and/or hives. They will also have elevated blood levels of a substance the immune system produces in response to particles to which they are allergic. They are IgE antibodies. Non-allergic asthma on the other hand, does not share these distinguishing characteristics.
When certain foods or medication such as aspirin trigger shortness of breath and wheezing, knowledge of those triggers can be a clue to the diagnosis of asthma. In contrast though, knowledge of various airborne particles, exertion, or an upper respiratory infection as triggers of symptoms are not good clues for making a distinction because they can all cause asthma attacks or exacerbations of COPD.
After careful consideration of many of the factors discussed, doctors oftentimes reach the conclusion patients have combined asthma and COPD. The importance of recognizing the coexistence of the two diseases is twofold. First of all, even though some of the medications used to treat both conditions are the same, some are different, and treatment guidelines for the two conditions also differ. Secondly, even though COPD and asthma are two separate sets of diseases according to their definitions, the World Health Organization does recognize mixed disease.
Evidence of world recognition of mixed disease is a specific code meaning both asthma with COPD and chronic obstructive asthma, as part of the looming ICD-10- CM coding system. The ICD-10 implementation should put to rest use of the archaic wastebasket term, asthmatic bronchitis. It will also increase the importance of differentiating between the two and identifying combined disease. It will also increase the importance of correctly identifying single and combined disease.