Having an allergy test


Blood testing

A blood test can be carried out to get an idea of how allergic a child is in general, called a serum total IgE. Blood tests can also reveal if a child is allergic to a specific thing, for example grass pollen or cats. I feel one has to justify taking blood from a child however, and would only do this if it will change management of that child.

Skin prick testing

Allergy testing can also be carried out using skin prick tests (SPTs). This is easier for young children than a blood test but can still be alarming for some of them. The validity of this testing in children under 3 years is not so good so I only do this in older children. We usually test for house dust mite, grass & tree pollen, cats and dogs, and aspergillus mould. We can add in other allergens if there is a particular reason e.g. the family has a pet rabbit. A positive result may increase the possibility of making a diagnosis of asthma, although is not diagnostic for it. Its other main use is if there is a pet at home, and we want to know if allergy to the pet is contributing to asthma symptoms.

How is it carried out? 
  • It is essential the child has not taken an antihistamine for 48 hours before the test otherwise it masks the results.
  • Small drop of the substances being tested are placed on the inside of the forearm.
  • A very fine pointed lancet is then used to touch the skin through the droplet to make a scratch on the skin. It is not nearly as uncomfortable as having a blood test.
  • After 15 minutes we look at the forearm and measure the red bumps (indicating allergy). The sizes are compared with the positive control which is a drop of histamine so the results are available immediately.
  • The bumps can be itchy but this usually goes by 30 minutes, if a child is bothered by it we can put some anti-histamine cream on the itchy areas.

Skin prick testing - on the left are the lancets and bottles of allergens used. On the right you can see a child's arm at the end of the skin prick test, with strongly positive results for the allergens placed at 3 & 6, mildly positive at 4 & 5, and negative at 7. The positive control is at 1 and the negative control at 2.