Auckland Allergy & Eczema Clinic
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Diagnosing Allergic Diseases

Diagnosing Allergic Diseases

Careful diagnosis of allergy improves your quality of life

The inconvenience of your hay fever, the embarrassment of your eczema and the time off work and school because of your asthma may be the most significant health problem in your life. Avoiding or minimising allergic triggers is an important part of your allergy and asthma care. Allergy testing helps to confirm the substances that you are allergic to, so that the appropriate advice and treatment can be given.

The increase in allergy awareness in New Zealand has created more demand for "Allergy Tests," as families see the need to diagnose allergy early. It is important to be aware of all the tests available, and emphasise the merits of the proven, useful tests, so that you will not be forced to resort to Alternative (Unproven) Allergy Tests. 

This article has been written to help you understand the way your specialist may assess you to diagnose allergic diseases.


Case History

A careful case history (your story) is the basis for the diagnosis and management of allergic diseases. The principle of history taking is the same for any medical problem (When, Where & What). You can prepare for your appointment with your specialist by considering these general points of particular importance:

  • Family history of allergies (Atopy)
  • Precipitating factors / things that provoke your symptoms
  • The home environment e.g. pets, carpet in bedroom, mildew
  • The work environment e.g. chemicals, irritants
  • Dietary factors e.g. hives within 1 hour of eating
  • Timing of symptoms e.g. day, night, seasonal, at work, at home
  • Hobbies and interests e.g. horse allergy in keen rider
  • Medication, especially less obvious over the counter preparations.

Skin Prick Test

This is the best test for allergy. The results are available within 20 minutes and your specialist will discuss the results during the appointment.

Skin prick tests can provide useful confirmatory evidence for a diagnosis made based on your case history.

Reasons for Doing a Skin Prick Test

  • To demonstrate atopy / allergic disease. People with atopy may suffer one or more of the characteristic disorders of asthma, eczema, or hay fever, multiple positive skin tests and a family history of allergic diseases. There is a good argument for performing skin prick tests as part of the routine work-up of all patients presenting with asthma, eczema and rhinitis — for proper management of these conditions the identification of allergenic triggers is essential before deciding on a long-term program.

  • Hives and angioedema. These conditions can sometimes be a sign of allergic disease.
  • Insect sting reactions. Testing to bee and wasp venom may be necessary if immunotherapy is contemplated.
  • Anaphylaxis. This test can find the cause. Testing should be timed at least 6 weeks after reaction.
  • Educational value. To provide a clear illustration for you that may reinforce verbal advice.

What Allergen Will We Test For?

Relevant inhalant (airborne) allergens will be included in a standard battery of skin prick tests.

Foods and other allergens will be selected on an individual basis, as dictated by your history.

Relevant Inhalant allergens in New Zealand in the skin prick test

  • House dust mite (D. pteronyssinus)
  • Temperate (springtime) grass pollens
  • Subtropical (summertime) grass pollens
  • Cat
  • Birch
  • Plantain
  • Moulds (Individually) – Alternaria, Aspergillus
  • Dog

Food allergens commonly tested by skin prick test

  • Milk
  • Eggs
  • Peanut
  • Tree nuts
  • Wheat
  • Soy
  • Fish & shellfish

Preparing for your Skin Prick Test

Antihistamines:

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Antihistamine tablets or syrups should not be taken for at least 3 days before testing as these interfere with the results. Some cold remedies and antidepressants can act like antihistamines. Please check with the clinic if you are taking these types of medications in advance of your appointment.

steroids:

If you are taking steroid tablets, please check with the clinic whether you need to change your dose. In some cases, you will be asked to reduce your dose in advance of the appointment.

Steroid creams should not affect this type of testing, but if you are unsure please check with our clinic prior to your appointment.

Nasal sprays & eye drops:

These medications do not affect the skin prick test and can be continued prior to your appointment.

Technique of Skin Prick Testing

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Your arm will be cleaned with soap and water or alcohol. The inner forearm is marked off with a skin-marking pen. The dots can be numbered to correspond to the number of the allergen being tested. Dots are usually at least 2cm apart.

Allergens are placed alongside the dots either using dropper from a vial or directly applied with the lancet. A sterile prick lancet is used to make a small prick and a new lancet is used for each allergen tested.

After this, the excess allergen is removed by laying a tissue on the arm (not by wiping). The test is then read at 15 minutes.

Reading of Skin Prick Test

If you are allergic to the tested allergen, a small itchy lump (wheal) surrounded by redness will appear within 15-20 minutes.

Reactions are assessed by the size of wheal produced. 


Intradermal Skin Tests

Intradermal skin tests involve injecting a small amount of allergen into the skin. This test is similar to skin prick testing. They are usually performed to help diagnose drugs and insect allergy. 

Preparation for this test is the same as the preparation for a skin prick test. 

If you are allergic to the tested allergen a wheal (itchy lump) reaction will appear within 20 minutes.

As for skin prick testing, reactions are assessed by the size of wheal produced. 

This procedure can only be performed by a specialist in allergy and is supervised more closely than skin prick testing.


Oral Food and Drug Challenges

Oral food or drug challenge is a test in which a food or drug is taken in gradually increasing amounts. It starts with a small amount of the food or drug and after a period of observation a slightly larger amount is taken. This allows your specialist to carefully evaluate for any symptoms that may signify a reaction. The amount taken increases until a typical portion or dose is eaten.

This test is performed under close supervision of your specialist and a nurse. In most cases, our specialists use this testing to demonstrate tolerance to a food or a drug if they suspect the allergy is "outgrown".

Why have a Challenge

The benefits of a food challenge include the nutrition and quality of life improvements from being able to expand the diet if the food can be successfully eaten.

Successful drug challenges, allow you to take a medication which may has a positive impact on the future treatment of illnesses or pain.

Preparing for your Oral Food or Drug Challenge

If you are taking antihistamines you will be asked to stop them 3-5 days before your challenge is performed.

You may eat and drink normally prior to the challenge.

The challenge will only be performed if you are feeling well. If you have infection, uncontrolled asthma or heart problems the test will be postponed.


Blood Tests

IgE

IgE (immunoglobulin E) are antibodies made by the immune system. Higher amounts can be found when the body overreacts to allergens. Levels are often elevated in cases of allergic disease.

Specific IgE

IgE antibodies are different depending on what they react to. An allergen-specific IgE test can show what the body is reacting to. This is the blood test version of the skin prick test. Your specialist may request this test to look for certain kinds of allergies.


Patch Testing

Patch tests are used to test how allergic you are when you come into contact with certain substances. Patch tests are different from skin prick tests, it tests the skin's reaction to a substance. A reaction to a substance by the skin is known as contact dermatitis. Patch tests don't use needles or lancets so you will not experience any pain when the patches are being applied.

During a patch test, your nurse or specialist will make up a series of small patches. Each patch contains a small amount of different substances that have been known to cause reactions in some people. The patches will be attached to your skin with medical tape. Most of the time, the patches are applied to your back or arm.

While the patches are on, you should avoid getting the patches wet. You cannot shower, take a bath, swim, exercise or do anything that might cause the patch to get wet.

Generally the patches are left on for two to three days. Once the time period is up, your nurse or specialist will remove your patches and look at your skin to see which substances your skin shows a reaction to. Reactions on your skin may appear as a rash, pimple or blister.

Usually, your specialist will have you return again four to five days after the original test. This is to see if you had a delayed reaction to a substance.

Once you know what causes you trouble, you know what to avoid. On the other hand, if you don't react to anything, your specialist will look in to other issues as the cause of any rashes you have.

 

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Some common Allergens (causing contact dermatitis) used in Patch Testing:

  • Nickel / Jewellery
  • Balsam of Peru / Perfumes
  • Dichromate in cement, leather, matches
  • Paraphenylenediamine (PPD) in hair dyes, clothing
  • Rubber chemicals in shoes, clothing, gloves
  • Colophony & adhesives in sticking plasters
  • Benzocaine in topical anaesthetics
  • Neomycin in topical medicaments
  • Preservatives in cosmetics, creams
  • Acrylics in artificial nails, cosmetics
  • Epoxy resins in glues
  • Formaldehyde in clothing, cosmetics, paper
  • Wool alcohol / Lanolin in cosmetics, creams

Unproven Tests in Allergy

There are several tests that claim to test for allergy. They use methods that are not scientifically proven and may lead to costly, unnecessary and harmful avoidance strategies. There are many types of unproven tests:

  • Applied Kinesiology – Muscle Testing

  • Cytotoxic Food Testing
  • Electrodermal skin test – VEGA Test
  • ELISA/ACT
  • Hair Analysis
  • IgG Antibodies to food
  • Iridology
  • Pulse Test

The use of these tests is not supported in New Zealand.