Skin Prick Testing

Skin prick testing is a way of diagnosing IgE mediated allergies. There is no absolute result and the testing must take into account what actually happens to the child with different allergens.


What is a skin prick test (SPT)?

SPTs are a way of testing for allergies that are IgE mediated - to read more about IgE mediated reactions, click here.

The test is looking for evidence of a reaction to a certain substance (usually a food) when it is put in the skin.

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How are skin prick tests performed?

Usually, the skin tests are done on the forearm or the back in small children. Once a decision has been made what is to be tested, small drops of fluid containing the test material are put on the skin. Then a lancet is used to prick the skin through the fluid. After 10-15 minutes, the size of the reaction is noted.

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Does it hurt?

SPT does not really hurt. At most it might be a little uncomfortable - like a scratch. The skin is hardly broken by the lancet.

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Does a positive skin prick test mean my child has allergy?

Not necessarily. The skin may react but your child may have no reaction at all when eating that food. The larger the reaction, the more likely it is there will be an actual allergic reaction to that substance.

For that reason, it is important to test only substances that you think your child may be allergic to rather than test everything.

To be sure, your doctor may want to do a formal food challenge in hospital to confirm the allergy. This is when your child is given the food in a safe environment so it there is any reaction, no harm will come to your child.

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What size skin reaction is likely to be associated with an actual allergic reaction?

It depends on the age of the child and there is no absolute size, but the following test results were all associated with a positive food challenge.

The wheal size refers to the size of the swollen area of skin. It may have some redness around it but don't measure that.

Less than 2 years of age
More than 2 years of age
 6 mm or more
  8 mm or more
 5 mm or more
 7 mm or more
 4 mm or more
 8 mm or more

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Are skin prick tests better than blood tests for allergy?

The SPT and the RAST (or EAST) blood tests give similar information. The higher the blood test result the more likely an actual reaction, just like the skin test. The advantage of the SPT is that you get an immediate result.

However, you can't test on skin that is covered with eczema so in those cases, the blood test may be better. The blood test may also be better if there have been severe reactions to food, such as peanut, in the past.

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Are there any precautions for SPTs?

Your child should not be taking anti-histamines - they should be stopped at least 5 days before the test. Some medicines need to be stopped for even longer. Check with your doctor.

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  • Sporik R, Hill DJ, Hosking CS. Specificity of allergen skin prick testing in predicting positive open food challenges to milk, egg and peanut in children. Clin Exp Allergy 2000; 30: 1540-1546.

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Last reviewed 16 May 2011

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Dr Maud MD

Dr Maud MD (MBChB, FRACP, FRCPCH), a specialist pediatrician, provides health information and medical advice for parents of babies and toddlers. Read more about Dr Maud.

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