Diagnosing allergies

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The specific triggers for your allergies need to be identified.
The specific triggers for your allergies need to be identified.

Allergies are treated by education, avoiding or reducing exposure to the allergen, medication and immunotherapy. If you suspect you have an allergy, it’s very important that you get the appropriate tests done before trying to avoid specific triggers. 

Many people have non-specific irritant triggers. For example, people with hay fever tend to react to cigarette smoke and very cold air, while people with eczema often react to heat, soap and rough fabrics. 

But allergy triggers differ from person to person. There’s no “one-size-fits-all” list of triggers for everyone who suffers from an allergy. For this reason, skin tests or IgE blood tests need to be done so that your specific triggers can be identified and avoided.

Medical history

The first step, before any tests can be ordered, is for your doctor to learn more about what you were exposed to and exactly what reactions occurred. This will help indicate whether the reaction was an allergy or not. If it was an allergy, your doctor will want to determine what kind of reaction it was, and whether it was mild or severe. 

If your medical history indicates that it wasn’t an allergy, no tests will be needed. On the other hand, if your history points towards a specific trigger, it can be very useful to do more tests. Note, however, that allergy specialists don’t advise doing tests for a large number of allergens “just to see” what you respond to.

Allergies are divided into: 

Immediate-type (IgE mediated) reactions, where the main problem is the release of chemicals from the mast cells when they burst.

Delayed-type (non-IgE mediated) reactions, which take longer to build up and where the main problem is the inflammation, swelling and damage to the body parts involved. 

These two types of reactions require different types of tests.

Testing for immediate-type reactions

There are two main tests that can be done for immediate reactions: skin prick tests and ImmunoCAP blood tests. These tests point to the presence of IgE antibodies. If the test results are negative, an immediate-type allergy is almost always ruled out.  

Because the tests show the specific allergy antibody, they’ll start to be reactive when you’re sensitised to a specific allergen. A test that’s “reactive” doesn’t automatically mean that you’re allergic; it could mean that you have the potential to be allergic. Sensitisation can lead to allergy, but in some cases the sensitisation is suppressed by other mechanisms in the body.

The greater the reaction is, the more likely it is that you’re truly allergic to that allergen rather than just being sensitised. The results of the test must be carefully compared with levels that have been shown to be truly positive, rather than just a little raised (reactive). In some cases, the test can’t give the final answer and other tests may need to be done.

Allergy tests are a guide to whether you have the potential to be allergic, but they don’t predict whether the reaction will be mild or severe. 

Skin prick tests

Skin prick tests (SPTs) use your body to show your doctor whether you have developed IgE antibodies to the allergens in the skin prick drops. SPTs are used to diagnose both allergies to allergens in the air (i.e. those that cause hay fever or asthma symptoms) as well as allergies to allergens in foods. 

An SPT can be performed easily in an allergy clinic. If you or your child is taking antihistamine tablets or syrup, you need to stop taking the medication 3–5 days before the allergy skin test is done, as it may interfere with the test.

The SPT is performed by placing a drop of the allergen on your skin. A lancet is then used to prick the drop through the top layer of the skin so that the allergen is introduced under the skin surface. The drop is then wiped away. This process is repeated for each allergen requiring testing.

After 15 minutes, your doctor will look at your skin for signs of a reaction. A negative reaction is when the skin remains normal. If your doctor sees a white wheal (also sometimes called a “hive” – a bump similar to that seen in an insect bite) surrounded by redness (also known as “flare”), he or she will measure it to see how large the reaction is.

The larger the reaction, the more likely it is that you’re truly allergic to that allergen rather than just being sensitised.

ImmunoCAP blood tests

The second test is called an ImmunoCAP blood test, which looks for the IgE antibodies to specific allergens in the blood. It therefore also shows sensitisation – the potential to have allergy; not the presence of allergy itself. The higher the ImmunoCAP result is, the more likely it is that you’re truly allergic to an allergen instead of just being sensitised.

The blood test is useful because it can be done even when you’ve recently taken antihistamines, and adults and children of any age can take the test. ImmunoCAP tests are available for hundreds of individual (specific) allergens: grass and tree pollens, mites, moulds, foods, insect venoms, pet allergens, cockroaches and more. 

The choice of allergen to be tested depends on the history of your illness, the history of any reaction(s), and the area in which you live. Some allergens are more common in coastal areas, while others are more common inland.

If you live in the countryside, your doctor may want to test for moulds that grow in granaries and thatched roofs, mites that grow in vineyards, and specific trees in the area. If you live near the sea, you may require testing for moulds that grow on seaweed.

Testing for delayed-type reactions

Tests for delayed-type allergy include blood tests (CAST, MELISA or BAT) and skin patch testing. 

These tests are highly specialised and used only rarely by general practitioners. They’re not “absolute” tests for allergic reactions, and need to be interpreted with care. 

Challenge tests

In cases that are uncertain, the allergist may recommend a challenge test.

For immediate-type reactions, a controlled incremental challenge test can be done. This is when the doctor gives you small, followed by larger, doses of the allergen at 15-minute intervals in a controlled setting (usually in a clinic or hospital). This can be done for foods and medicines, but isn’t done for insect allergies.  

For delayed-type allergies, proof of a specific allergen causing symptoms relies on removing that allergen for a few weeks. If symptoms go away, it’s then critical to “re-challenge” you with the allergen to prove that symptoms come back. Note that an elimination challenge test is NOT the same as a challenge test done for immediate-type reactions. 

Alternative and complementary allergy tests

There’s no evidence that ALCAT, Vega testing, pulse testing or kinesiology are of any benefit in diagnosing food allergy. If you use these tests, you may spend a lot of money, only to receive an incorrect list of allergens to avoid. This could meant that you compromise your nutrient intake and health without good reason to do so.

Reviewed by Prof Mike Levin, Head of Division of Asthma and Allergy at the University of Cape Town: MBChB; FCPaed; MMed; PhD Diploma Allergology; EAACI UEMS Exam in Allergy, FAAAAI, FACAAI. March 2018

Image credit: Shutterstock

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