Asthma: Diagnosis and Management ____________________________________________________________
ASTHMA CHALLENGE TESTS When an asthma condition is difficult to confirm or treat, asthma challenge tests may be undertaken to verify the diag- nosis. Of course, any challenge test is potentially dangerous, as there is a risk that the test may induce a serious asthma emergency or unpredicted delayed reaction. So, preferably, challenge tests should be performed in a hospital setting under controlled conditions. There are two main challenge tests that are useful in assessing a patient for a possible asthma diagnosis. The first is an inha- lational or chemical challenge (i.e., direct challenge), whereby patients inhale a small amount of either a suspected asthma trigger or one of two chemicals: histamine, which occurs naturally in the body and may induce asthma-like symptoms, or methacholine, which is known to cause airway constriction only in people with asthma. After exposure, spirometry is per- formed. If a reaction does not occur initially, the concentration of the substance is increased and spirometry is repeated. The procedure is repeated until it is determined that there is no sensitivity or until FEV 1 decreases by 20%, which would be indicative of asthma [9; 13]. Second, if respiratory distress increases with exercise, an exer- cise challenge (i.e., indirect challenge) may be recommended. Spirometry is performed before, during, and after the patient engages in moderate-to-strenuous activity, such as running on a treadmill or riding a stationary bicycle, in a controlled laboratory setting. If peak flow or FEV 1 drops more than 12% to 15% during or after the activity, the diagnosis will most likely be exercise-induced asthma [9; 13]. ALLERGY TESTING Because the comorbidity rate associated with asthma and aller- gies is so high, it is advisable to engage those patients suspected of having asthma in standard skin and blood tests to determine if they are atopic. The most common allergy tests involve introducing the suspected allergen into the skin’s surface and monitoring the area for a reaction. This skin testing can be expensive, time consuming, and, if conducted improperly, results may be misleading. Nonetheless, skin testing can be useful in confirming suspected allergies. The three types of skin tests are prick, scratch, and intradermal, each of which involves adding small amounts of a given allergen to the skin, either percutaneously or intradermally. The test site should be assessed after approximately 15 to 20 minutes to determine if a reaction has occurred. A positive allergic reaction is char- acterized by a wheal at least 3 mm in diameter greater than the negative control. A few patients may experience delayed anaphylactic reactions to even the smallest amounts of aller- gen; untreated, these reactions can result in asthma attacks of increasing severity, especially in sensitive individuals [9; 10; 14]. Therefore, many physicians and allergists favor elimina- tion diets over skin tests for differentiating food sensitivity.
In addition to elimination diets and skin testing, there are various blood tests that may be used to assess allergy or allergic reactions in a patient. These tests are able to calculate the pro- portion of the antibody IgE in the body. In infancy, IgE levels tend to be low, rising gradually over the following decades and decreasing beginning around 40 years of age. Lowest levels are recorded when an individual is 70 years of age. Therefore, high IgE levels can imply that allergies may be triggering the asthma. Elevated IgE levels in infants and young children are one test used to help predict future allergies. Although the analysis is fairly common, it is rarely used as a sole indicator of allergy or allergic asthma. A group of tests, the radioallergosorbent test (RAST), the multiple allergosorbent test (MAST), and the fluorescent allergosorbent test (FAST), evaluate blood levels of IgE for sensitivity to specific allergens [9; 10; 14]. Although the connection between asthma and allergies is compelling, it is vital to remember that not all people who develop asthma have allergies. Whereas allergies play a part in asthma for 80% to 90% of children, the figure is thought to be lower for adults [14; 24]. Nevertheless, it is worth inves- tigating the possible role of allergens in asthma because, quite simply, knowledge of allergic triggers associated with asthma will allow patients to avoid the harmful agents and better manage the condition. Allergen and asthmagen avoidance is one of the most effective treatments for asthma and allergy patients [9; 10]. DIFFERENTIAL DIAGNOSIS Because the signs and symptoms of asthma are similar to sev- eral other diseases and disorders, it is important to rule out other conditions that may have a similar outward appearance of asthma. This is particularly imperative for patients unable to express symptoms or history verbally. In particular, panic disorders, physical airway obstructions, congestive heart failure (CHF), GERD, and other pulmonary conditions, such as COPD, pneumonia, and bronchitis, may either exacerbate or mimic the signs and symptoms of asthma. Shortness of breath, decreased exercise tolerance, chest tight- ness, and wheezing may occur with any of these conditions. Although not all individuals with asthma wheeze, it is one of the characteristic symptoms of asthma, occurring either during episodic attacks or quite regularly, depending on the severity of the condition. However, there are several other potential causes of wheezing that should be investigated as part of the diagnosis process. For example, wheezing in children may be attributed to acute infections, including bronchiolitis or pneumonia. If these diseases are suspected, chest x-rays and/ or blood gas tests may be ordered.
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