Allergy testing

Allergy tests are performed to either confirm a suspected allergy, or to exclude a specific allergy as the cause for the symptoms. There are many allergy tests, but only two have been scientifically proven: the blood test and the skin prick test. These tests are complex and require a skilled professional (often only doctors or dieticians with a special interest in allergies) to interpret the results.

What is a "blood Test"?

A small sample of blood is taken from the arm and sent to a laboratory for testing. The blood test measures how many antibodies (called IgE) there are to a specific allergen (a food or substance that is causing the symptoms) in the blood. The IgG and ALCAT tests are also blood tests, but are not scientifically proven tests and are not recommended.

The result is reported as a numerical value or "class" where 0 indicates a negative result and 6, a strongly positive result. Over 400 different specific IgE allergen tests, which encompass a range of pollen, food and occupational allergens, are commercially available.

If the result is positive, it does not necessarily mean that you are allergic to that substance. However, the higher the IgE level to a substance, the more likely you will be allergic to it. Your doctor or dietician will need to take various factors, such as your type of symptoms and history of allergy to determine whether the substance is the likely cause.

If the result to a specific substance is negative, it does not necessarily mean that the person is not allergic to that substance either. If you have been avoiding the food for a period of time, the amount of antibodies to that particular food becomes lower. Also, if you have an allergy that does not work through the specific IgE mechanism, the blood test would also not pick it up. This type of allergy is more difficult to diagnose and your doctor or dietician would need to use other methods such as an elimination diet and challenges. Ask your doctor or dietician for more details.

What is a "skin prick test"?

The skin prick test is just that. A drop of a liquid extract of the allergen is placed on the skin of the inside of your arm (or the back). The skin is then lightly pricked with a device such as a needle or lancet through this droplet so as to just puncture the skin. It is not painful; one can hardly feel it.

Test results are available within 15 minutes of testing. A positive skin test is indicated by the development of a "wheal-and-flare" reaction where the skin was pricked. The "wheal-and-flare" reaction looks like a mosquito bite; the larger the size of the "mosquito bite", the more likely that you are allergic to that particular substance.

Skin testing is simple, quick to perform, relatively safe and more cost-effective. The results are also immediate. There are around 25 allergens that you can choose from, but usually 3 - 6 are tested at a time. Although the amount of allergen introduced into the skin is very small, there have been rare reports of anaphylactic (life-threatening) reactions following skin prick testing, especially with allergens such as peanut and bee venom.

As with the blood test, the interpretation of the results is not so straightforward. Positive tests indicate that IgE antibodies are present but do not, in isolation, prove that a reaction will occur upon eating the food. In fact, people who 'outgrow' their food allergy usually continue to get a positive test result to the food for many years.

Which test should I make use of to diagnose my allergy?

Each test has its own advantages and disadvantages.

Advantages of the blood test

• Blood tests are accurate, convenient and test a broader range of allergens than the skin tests.
• The skin test's reliability can vary from one manufacturer to another, compared to the blood test which is standardised.
• Antihistamines may interfere with skin tests, so blood tests are more appropriate where antihistamines cannot be discontinued for whatever reason.
• For people with extensive skin disease, skin testing may not be possible.
• Periodic blood testing can be useful in predicting when a child has developed outgrown an allergy or successfully avoiding the allergen. This is difficult with skin testing.
• In people with a history of life-threatening reactions, blood tests are preferred, as contact with even a droplet of the allergen on the skin (as in the skin test) can result in a reaction, especially in allergens such as peanut.

Advantages of the skin test

• The blood test doesn't give an immediate result, as with skin tests.
• Some allergens in foods such as fruits and vegetables are destroyed during the preparation of the extracts for blood tests. Testing blood samples for these allergens will therefore not be as accurate. Skin tests may be of benefit here as the fresh food can be used on the skin. (Don't try this at home though!)
• Since the blood test involves drawing blood, it costs more than the skin tests.