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Allergy Tests for Atopic Eczema

Parents often ask us if their child is allergic to something. This is because they believe that avoiding something can clear their child's atopic eczema.

"Allergy tests" are used to further research into eczema but as a guide to the treatment of eczema, they tend to confuse both doctors and patients. The allergy tests used in research studies tell us something about substances in the blood but unfortunately, they will not tell us if your child's eczema will respond by avoiding these substances.

The best test of whether your child's eczema is caused by a particular substance is your own observation. For example, itchy patches and swellings developing in the skin after stroking a long haired cat accompanied by sneezing and runny eyes are a fairly reliable sign that your child is allergic to that particular cat. Food allergies are sometimes more difficult to identify and if you suspect that a particular food is worsening your child's eczema, then exclude that food completely for 6 weeks and at the end of that period, give your child the food again to see whether it brings on a definite reaction. Test only one food at a time and keep a diary of reaction. Remember that atopic eczema is a condition which comes and goes quite rapidly and if you are unclear whether your child's eczema has really worsened during the challenge then repeat the whole process again. This is time consuming but it is the only reliable way of finding out more about a possible food allergy in your child.

If you would still like to know more about allergy tests in atopic eczema, then please read on.

RAST Tests

This is a blood test that tells us if your child has antibodies in the blood which could react to common substances which can cause allergies. There are 3 reasons why doing a RAST test is not very helpful in guiding your child's treatment: a) what happens in the blood may have little to do with what happens in the skin, b) what happens in the blood may reflect another allergic condition such as asthma or hay fever and have nothing to do with your child's eczema and c) many people who do not have eczema, asthma and hay fever have high levels of these antibodies in their blood.

For this reason, a positive RAST test to a particular substance is not very helpful in predicting whether avoiding that substance will help your child's eczema. On the other hand, a negative RAST test is sometimes helpful in suggesting that your child is not allergic to that substance.

Skin Prick Tests

Substances that may cause allergies are made up into solutions. Drops of these solutions are put onto the forearm and the skin is pricked with a tiny needle. A red swelling will develop if your child's skin has reacted to any of the solutions used.

This test is not a helpful guide to eczema treatment in your child because a) what happens in the prick tests may have very little to do with your child's atopic eczema, b) the skin prick test result may reflect another allergy linked with asthma or hay fever and c) positive skin prick tests are common in people who do not have any obvious allergic disease. Skin prick tests are used widely in purposes but like RAST tests, they are not a useful indicator of how your child's eczema will respond to avoiding a particular substance.

Patch Tests

Sticky patches containing various substances are placed on your child's back. This type of test is used to investigate people with suspected contact dermatitis e.g. someone who has hand eczema caused by an allergy to rubber through wearing rubber gloves. The rubber substance will show up as a reaction on the back 48 to 96 hours after placing the patches on. Avoiding that substance in the future usually helps in clearing the problem.

Unfortunately, this type of contact allergy is not very common in atopic eczema. The commonest reason for worsening of your child's hand eczema are the irritant effects from contact with water, soap, sand, saliva, foods and cold wind. These effects are not allergic in nature.

More Information?

Please discuss this with the eczema team who will be happy to answer your questions.

© Professor Hywel C Williams, Sister Sandra Lawton Staff Nurse Sue Newham Queen's Medical Centre, Nottingham, 1995.