Skin-prick testing
Skin-prick testing is a simple, quick and inexpensive test, which can be performed using a wide range of allergens, and results can be available within 15 min. The principle of the test is that if the patient is allergic to a compound they will have produced specific immunoglobulin (IgE) to that compound. This immunoglobulin will recognize the compound when it is administered. Skin-prick testing should relate to the clinical history of the patient, which should indicate the allergens selected. In the test, a small amount of the allergen is placed under the skin. If the patient is allergic to the allergen, the IgE that sits on top of the mast cells will recognize the allergen and cause the mast cells to degranulate. Mast cell degranulation causes increased blood flow to the area, causing it to become red, hot and swollen.
How the test is performed
• The test is generally carried out on the skin of the inner forearm The arm is labelled in advance with numbers indicating specific allergens, together with a positive control (histamine) and negative control (saline) • A drop of allergen is placed on the skin beside the appropriate label and pricked gently into the epidermis using a lancet • A new lancet should be used for each allergen • After two minutes the area can be blotted with a clean tissue • The test sites are read at 15 mins for the presence of erythema (redness) and weal • The size of the weal is measured and a weal 3mm> the negative control is considered positive • Large weals at all sites or failure of all tests including the positive control site suggest the need for an alternative test such as specific IgE.
Nursing considerations
• The patient should be advised not to take antihistamines before the test as the result will be unreliable • Patients should not be on high-dose oral steroids at the time as the test results will be unreliable • It is safer to avoid skin-prick testing in patients with a past history of anaphylaxis, or with moderate to severe eczema • The patient should be warned that a positive response to the allergen may be uncomfortable as the skin swells and gets warm • Very rarely, a patient may develop a severe reaction such as an anaphylactic response to the allergen • Because of this risk, resuscitation equipment and trained personnel should always be available when skin tests are being performed.