MEDICAL, LABORATORY AND OTHER TESTS FOR IDENTIFYING ALLERGY: INTRA-DERMAL TESTS AND PATCH TESTS
In intra-dermal tests, a higher dose of allergen extract is introduced just under the outer skin using a syringe. A similar weal and flare response within 10-15 minutes indicates a positive reaction. In addition, you may get a late skin reaction – a raised red swollen bump around the test site – about five or six hours later. Sometimes there may be a late skin reaction on the following day.
The infra-dermal test is much less commonly used in the UK than the skin-prick test. It can be painful, it has a slight risk of adverse reaction, and some people feel unwell on testing. It can, however, be useful in that it can detect positive reactions where skin-prick tests have previously been negative.
Patch tests are used to confirm a diagnosis of contact dermatitis – a delayed allergic skin reaction to something you have touched. Patch tests can be very useful in identifying specific things that you can then avoid. Small patches containing a range of common allergens, mostly chemicals, are attached to the skin, usually to the upper back. The sites of the tests are marked on the skin. The patches are left there for 48 hours, and then removed.
The sites are examined for reaction and then left unwashed for a further 48 hours, when the sites are examined once more. A raised red bump at the site of the allergen is an indication of positive reaction. False positives and false negatives can result, so once again patch tests are used mainly to corroborate a case history. You can use patch tests to test specific things you suspect – for instance, a fabric, a leaf or a chemical you use at work or school. You can also do a home version of the patch test for yourself.
Anti-histamine drugs block the release of histamine and can interfere with skin test results. Such drugs should not be taken for several days before testing.
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