A skin prick test is a simple and rapid test which can tell whether one is allergic to an inhaled allergen (such as house dust mite, pollens, animal dander, feathers, moulds) or food allergens. It is the most commonly used test for the diagnosis of allergy.

1. Allergens for skin prick testing are watery solutions stored in small bottles.  The doctor will chose which allergens to test based on the information received from the patient while taking the clinical history.

2. One small drop of the allergenic solution is placed on the skin of the forearm. Besides the allergens, a positive control (histamine), and a negative control must always be used with every patient.

3. The skin is then pricked through the drop with a sterile lancet with a very fine tip, which ensures that only the superficial layers of the skin are penetrated. So there is almost no pain and not a single drop of blood.

4. The name of the tested allergens can be written on the skin, to ensure that there will be no confusion when reading the results. Here the first allergen is house dust mite (HD) the second one is grass pollen (Grass), third is the negative solution      (-ve) and the fourth is the positive solution test (+ve). The excess of allergen solution is wiped off the skin with a paper tissue.

5. Results are red after 15 minutes. Reactions induced by the allergens must always be compared with the positive and negative tests. The positive control should induce a red itchy swollen area (a weal) of at least 5 mm in everyone while the negative test should not show any reaction at all. A positive reaction to a tested allergen is indicated by the presence of a red weal more than 2mm greater in diameter than the negative test and more than ½ of the size of the reaction induced by positive control. This patient has a strong positive reaction to house dust mite and no allergic reaction to grass pollen.

Usually the itch accompanying the positive reaction subsides in 30 minutes to 1 hour while the redness and swelling may persist for a few hours before wearing off completely. Skin prick tests do not  leave any permanent marks on the skin.