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Tests Used to Diagnose Allergies

Instead of skin tests, your pediatrician or allergy specialist may order a blood test that has various names, including specific IgE blood test, in vitro IgE test, and radioallergosorbent test (RAST). (When the test was first invented years ago, it was called RAST, based on the specific way the test was done in the laboratory. Even though that technique is no longer used, the name RAST has kind of hung on.)

The specific IgE blood test is especially useful if skin tests cannot be done because, for instance, a child has eczema over much of his body or cannot be taken off medication that interferes with skin testing. This blood test shows specific sensitivities, as skin tests do, but does so by detecting the presence of allergy antibodies circulating in the blood. If antibodies are in the blood, it usually means the same antibodies are also in other tissues.

The method is not quite as versatile as skin testing because certain extracts are not available for measuring specific IgE using this technique. For example, a specific IgE blood test cannot be used to detect sensitivity to medications and is rarely used to detect insect venom allergy. However, the specific IgE blood test, in general, is adaptable and sensitive enough to detect a wide range of allergies.

The procedure costs more per test than skin testing. It requires only a few minutes of the patient’s time to draw a blood sample and there is no risk of any allergic reaction. The results take from 1 to 5 days, whereas skin test results are available immediately.

Radiographs and Imaging Tests

While sometimes useful, radiographs (x-ray films) are not essential for diagnosing asthma or allergies. In fact, people with asthma usually have normal chest radiographs. However, chest radiographs are sometimes done to make sure children do not have other conditions that can mimic asthma.

Sinus infection can produce symptoms similar to those of respiratory allergies, and children who have respiratory allergies are prone to sinus infections. Your pediatrician may order an imaging test to see if your child simply has a prolonged or recurrent infection, or whether a sinus infection is complicating his allergies.

An imaging test can be done the old-fashioned way, with a radiograph of the head, or it can be performed by computed tomography (CT). A CT scan is more sensitive than a radiograph and shows finer details of the anatomy of the sinuses, which can help your pediatrician decide on the best way to treat your child’s sinus problem.

Finally, imaging tests can sometimes help your pediatrician identify the reason your child snores or has a permanently stuffed-up nose. A radiograph of the upper neck area can show if the stuffiness is caused by enlargement of the adenoid tissue, which sits in the upper throat just behind the nose.

Lung Function Tests

If your child has symptoms indicating possible asthma, your pediatrician or asthma specialist may perform tests to evaluate his lung function. Lung function tests are performed in your pediatrician’s office or a pulmonary function laboratory where special equipment is available. An instrument called a spirometer is used to measure how much air your child can breathe out, as well as how fast the air flows. The technician will place a clip over your child’s nose to prevent air escaping from the nostrils.

The technician will then ask your child to perform breathing maneuvers into a mouthpiece attached to a pulmonary function monitor. The maneuvers aren’t difficult or painful. All your child has to do is take a deep breath, then breathe out forcefully through the mouthpiece. Instead of using a spirometer, the doctor may ask your child to blow into a simpler device called a peak flow meter. Your pediatrician usually has your child perform the lung function test at least 3 times at a sitting to make sure results are consistent.

If lung function testing shows that your child cannot blow air out fast enough, your pediatrician may perform further tests for asthma. Your pediatrician may give your child a dose of bronchodilator medication to see if there is a change in airflow. If airflow is normal or improved after the medication, the result strongly suggests that asthma is present.

Sweat Test

Cystic fibrosis is an inherited disorder that involves many body systems. It causes symptoms in the respiratory and digestive tracts that can mimic those of asthma and allergies. A child with cystic fibrosis may have asthma or allergies, as well. Your pediatrician may order tests to measure the levels of certain minerals in your child’s sweat (commonly called a sweat test). If the results indicate cystic fibrosis, further tests will be done to confirm the diagnosis.

Last Updated
Guide to Your Childs Allergies and Asthma (Copyright © 2011 American Academy of Pediatrics)
The information contained on this Web site should not be used as a substitute for the medical care and advice of your pediatrician. There may be variations in treatment that your pediatrician may recommend based on individual facts and circumstances.
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