Performing the prick test
How is the prick test performed? Make sure the skin is healthy, clean and dry. Special preparations are not required, except that the skin should be allowed to 'acclimatise' for a while in the event of extreme outdoor temperatures. The allergen extracts are placed on the skin (for instance, on the inside of the forearm. The various drops should be spaced at least 4 cm apart to prevent the various extracts from mixing. Make sure the dropper does not touch the skin. The method of pricking is described below. A new lancet must be used for each test solution. The result becomes visible after 15 to 20 minutes.
Identical negative and positive tests
The negative test and positive test are performed in an identical manner. After pricking, any excess test solution is carefully dabbed off. The forearms should remain exposed, and the patient must under no circumstances scratch them.
Positive and negative test
Besides the prick test, a positive and negative test are also performed. For the positive test, a drop of histamine is applied underneath the series of allergen extract drops, to which the patient must react. A negative test result may be an indication that the patient has used allergy-suppressing medication, such as antihistamines. This means that the skin test is unreliable and must be repeated at a later time.
The negative test merely contains a buffer solution and the patient should not react to it. If the patient does react, it is a sign of hypersensitivity to any of the ingredients of the buffer solution or he or she is reacting to the prick itself. In that case, a laboratory test (RAST) will be required to diagnose the allergy.
How is the outcome of a prick test interpreted? (these images are not to scale)
According to the guideline of European allergologists (EAACI), a prick test is regarded as positive if the weal has a diameter of at least 3 mm.
For documentation purposes, the following details need to be recorded:
* Patient's name
* Test date
* Tester's name
* Allergens tested
* Prick test results
How can the use of symptom suppressants influence the results of a prick test?
The use of symptom suppressants may influence the results of the prick test. This becomes visible as the positive test does not react or reacts only slightly. The test is then useless and will have to be repeated. The use of symptom suppressants does not influence the results of a laboratory test (RAST). The use of symptom suppressants must therefore be discontinued for a certain period prior to the prick test. It is recommended to discontinue their use as follows:
* Modern oral antihistamines: 2-7 days in advance
* Oral corticosteroids: 1 week in advance, in consultation with the doctor
* Tricyclic antidepressants 7 days in advance
* Local corticosteroids do not influence the test and their use does not have to be discontinued, as long as they are not administered to the forearm.
