The Prick test and the Patch test are skin reaction tests used to ascertain (diagnose) allergies. They have an undisputed validity as long as they are performed and evaluated correctly.
The Prick test it is the most common allergy test to identify the presence of food and respiratory allergies. It is usually prescribed to identify allergies to allergens present in various sources such as:
- animal hair
- dust mites
- insect poison
The Patch test (or skin test) is mostly used to ascertain contact dermatitis and is used to evaluate which substances (haptens) are capable of triggering an allergic reaction. Among the substances that cause most of the reactions in the patch test should be considered:
The Prick test it is a quick and painless examination, it can be performed on both adults and children. The test serves to highlight the presence of IgE antibodies responsible for the manifestations of allergy.
The test consists in the application of a drop of each sensitizing substance (allergen) on the skin of the forearm (in children it can also be carried out in the upper part of the back); the skin, then, is lightly pointed with a sterile lancet in correspondence with each drop, to allow the components of the allergen to come into contact with the superficial layers of the skin (epidermis). To check if the skin reaction is correct or excessive (as occurs in allergies) specific substances are applied such as histamine, glycerin or a saline solution.
In most cases histamine causes a skin reaction, if this does not occur the test may not reveal the presence of an actual allergy. If, on the other hand, a skin reaction occurs after the application of glycerin or a saline solution , which in most people do not cause any reactions, the test results will need to be interpreted with caution to avoid a false diagnosis of allergy.
After about 15 minutes of waiting, the skin is examined to evaluate the reaction to allergens which manifests itself through the appearance of swellings (wheals) that cause itching, surrounded by a red halo (erythema), very similar to those caused by the sting of a mosquito.
The Patch test it is usually performed to check whether a particular substance causes an allergic skin reaction (contact dermatitis); it is a painless examination that is carried out by applying, on the skin of the back, patches containing the allergens to be evaluated. These are left on the skin for 48-72 hours, during which it is necessary to avoid getting wet and, possibly, limit sweating by avoiding physical activity and hard work to ensure that the patches themselves do not come off the skin.After 48 hours, the doctor carries out a first reading to check for any skin reactions (erythema, edema, papules or vesicles). The final reading is performed after 72 hours.
Unlike the Prick test, the Patch test it takes longer because it is a test that tests the sensitivity reactions mediated by cells (and not by antibodies) that need at least two days to take place.
The Patch test it is especially useful to ascertain contact dermatitis and, as regards some aspects of food allergies, to verify the presence of allergy to nickel and food additives contained in prepackaged foods (e.g. jams, fruit juices, preserves, products in box etc.).
No special preparation is required to undergo skin reaction tests but it is essential to inform your doctor if you are taking any medications as they may alter the test results. In particular, the doctor should be warned if you are following antihistamine and / or cortisone therapy.
Skin reaction tests must be carried out under the supervision of medical staff who will take care to report any contraindications to their execution (for example, ascertained pregnancy).
The interpretation of the results of skin reaction tests is carried out by a doctor who, in the presence of skin reactions (redness, edema, blisters, wheals), assesses their intensity, identifying which substances are responsible for the reactions.
Skin reaction tests do not give complete certainty of the causes that triggered the allergy since so-called false positive or false negative results are always possible.Furthermore, since there is a possible relationship between an allergic reaction and other diseases, in both cases an accurate medical examination is essential, which also allows to know the clinical and family history, in order to formulate the right diagnosis.
NHS Choices. Allergies (English)
Mayo Clinic. Allergy skin tests (English)