Diagnostic tests

Allergy testing is done either through a skin prick test or through a blood test. These tests are used to determine if a patient has allergic antibodies called IgE that recognize specific allergens such as foods, medications, insect venoms, pollens, mold spores, animal dander. These IgE antibodies bind components of the allergens (certain proteins) and then activate allergy cells called mast cells. The mast cells release a substance called histamine which causes hives and itchiness among other allergy symptoms. Hives look like mosquito bites: red, raised, itchy bumps that are transient on the skin.

Skin prick testing

A small plastic stick with sharp ends is used to first take up concentrated protein extract (food, tree pollen, grass pollen, mold spores, animal dander) or a fresh food sample and then it is used to prick the skin. The skin breaks and allows the allergenic proteins to come into contact with potential IgE antibodies. Once these antibodies recognize the specific protein they activate allergy cells called mast cells some of which are found in the skin. These cells release a chemical called histamine which creates a hive.

A test is positive if a hive appears in the area of the specific protein antigen being tested. To interpret the test, a negative control is used, which is the diluent used to dissolve the proteins and does not contain any allergens. Some patients have a condition called dermatographism (“writing on skin”) which is when hives appear on the skin due to pressure. The negative control allows us to ensure that the test is not positive only because of the pressure applied. A positive control, histamine itself is applied as well. This is useful since antihistamines and other medications such as: anti-nausea medications, antidepressants, antipsychotics and sleeping pills are known to interfere with the skin test and decrease or inhibit the skin test reaction. (Please note, patients should not stop these medications without consulting the prescribing physician.) The test is read in 10-20 minutes. The hives that develop, disappear over the next 30 minutes to 1 hour. Non-sedating antihistamines may be taken to help with the itch. The test is generally safe and is done in patients of all ages.

Having evidence of specific IgE antibodies does not always correlate with an allergic reaction. Allergists use skin testing as a tool but, alone, a positive skin test is not sufficient to determine an allergy. Some patients have the antibodies but do not have symptoms. The positive test increases tells us that there is an increased probability of symptoms. We do not panel test foods as there is a chance of false positives which is higher in patients with eczema.

A negative skin test is very good at ruling out an allergy. If the test is negative then the likelihood of an allergy is very low. If a patient is able to tolerate the food on a regular basis then testing is not performed as the food is being eaten and tolerated.

Intradermal skin testing

Testing for a penicillin allergy or for insect venoms is done through intradermal skin testing. This test involves very using small needles to inject the testing material just under the skin to form a bleb. The test is read 10-20 minutes later and looks for redness, itchiness and an increased bleb size.

Serum specific IgE

This is a blood test that looks at the amount of IgE antibodies present that can recognize certain protein. This is available for certain foods, insect venoms (been, wasp, hornet, yellow jacket), pollens, mold spores, and animals. Again, an allergist will use this as a tool to determine the likelihood of symptoms being due to an allergic reaction. It should not be ordered in patients without allergic symptoms, as the interpretation of the test is dependent on its correlation with the clinical history. This test is different from IgG testing to foods. This test is not offered by allergists as the IgG antibodies are not allergic antibodies, and quite the opposite, it generally means your body has seen a particular food many times. The IgG testing for foods is not recommended. It may lead to inappropriate diet restriction which is particularly harmful in children and may lead to malnutrition if a very restrictive diet is recommended. In young children, if a particular food is avoided for a period of time, an allergy may develop. Please see the Food Allergy section for your information.

Oral food challenge

Sometimes, allergist s in discussion with patients may decide to try small amounts of a food to prove that the patient is not allergic to it or has outgrown the allergy. This is done after appropriate consultation and determination of risk. Challenges involve eating increasing amounts of a food at intervals of 15-30 minutes in the Allergy office which is prepared to treat anaphylaxis if it does occur. The risk of doing a food challenge is determined by the allergist and discussed with the patient, as the risks and benefits are assessed.

Drug oral challenges

When there is a low likelihood of an allergic reaction to a medication, that medication is given in the office and the patient is monitored for 1 hour. If no reaction occurs, they are then cleared of the allergy label. The risk of an allergic reaction to the medication is then deemed the same as the general population. In the case of penicillin, many patients were labelled with the allergy in childhood that are not allergic. Most hives in childhood occur due to infection and at times children receive antibiotics for infections. Even if there was an allergy to penicillin in the past, 80% of patients lose the allergy after 10 years. Studies have shown that it is safe to do amoxicillin (a type of penicillin) or penicillin oral challenges in children and adults who are identified as low-risk. There are other immune reactions to medications that need to be considered based on the reaction type. An evaluation by an allergist may also involve medication skin testing with skin prick or intradermal skin test or both. Penicillin allergy de-labelling is important, as it can prevent side-effects of using other medications, and prevents the development of drug-resistant microbes.

Spirometry

If you have asthma or there is suspicion of an asthma diagnosis, a breathing test may be ordered. The testing may be done in certain offices or at hospitals. It involves blowing really hard into a machine that can measure how fast the lungs are able to expel air out and compare it to statistic data from a population with normal lungs of the same height and weight as the patient. At times, Ventolin is given and if a significant improvement in the lung function is seen, this may be suggestive of asthma.

 Dr. Ana-Maria Bosonea