Skin Tests (Prick Test and Intradermal Test)

What is it for?

Skin tests are ordered when there is a suspected allergic reaction or disorder after t a medical history has been taken (through questions) and the patient examined. Skin tests, thus, give us an objective confirmation of sensitisation to an allergen, although the relevance of that sensitisation has to be interpreted with the patient’s history borne in mind, so that the appropriate advice on avoidance and treatment can be given. 

How are they done?

There are two main types of skin tests: 

• Skin prick tests: a drop of a substance is placed on the skin, which is gently pricked through the drop with a small, single-use lancet.
• Intradermal tests: a very small quantity of a diluted substance is injected into the outermost layer of the skin. 

The results are read after 15-20 minutes. If the patient is “sensitised”, the substance will induce a local reaction with itching, redness, swelling, etc. This reaction is compared with tests done with physiological serum (negative control that should not cause a reaction) and histamine (positive control that should cause a reaction).

The patient should not take antihistamines (anti-allergy medications) for 5-7 days prior to the test.

Risks

The risk with these tests is very low. Only in extremely allergic patients, and usually while testing medications, is there a certain risk of their causing a serious and generalised allergic reaction.

Alternatives

In some cases, blood can analysed to assess its sensitisation.