The prick test is a simple, safe and inexpensive allergological diagnostic investigation; for this reason, in general, it represents the first instance examination when faced with the suspicion of an allergy. The prick test is also carried out in the pediatric field starting from one year of age, while in younger children it is of little use due to poor skin reactivity.
The prick test is frequently used to screen for allergens of food and inhalation origin. For the identification of skin allergies, such as nickel , the so-called patch test is used instead .
How is the prick test performed?
- The prick test is performed on the skinof the flexor surface of the forearm; in case of eczema or skin lesions in this area, and in younger children, the examination can be performed on the skin of the back.
- The points under investigation are marked with a felt-tip pen. A drop of diluted allergen extract is then placed on the corresponding skin.
- This drop is crossed by a sterile lancet (a tiny pin), made to penetrate about 1 mm into the epidermis(prick in English means to prick).
- After one minute the drop is removed with a piece of absorbent paper.
- After 15-30 minutes the cutaneous response to the prick test is observed. The examination is considered positive if a whealwith a diameter equal to or greater than 3 mm appears , surrounded by an erythematous halo (redness), generally itchy.
Is the exam safe, does it cause pain?
The prick test is a safe, painless and well tolerated diagnostic investigation. The itching caused by an allergic reaction usually lasts no more than an hour. Only on rare occasions can serious adverse reactions occur (malaise, breathing difficulties, nausea and vomiting , pressure drop up to shock).
It is therefore recommended not to perform the prick test in patients with a history of anaphylaxis . People at high risk of allergic reactions should perform the test in a hospital, with health personnel equipped with first aid drugs, such as cortisone, antihistamines and adrenaline .
The reliability of the prick test is strongly influenced by the operator. For this reason it is essential to respect the following directives:
- Through the prick test it is possible to test a large number of allergens; in this case the single drops must be applied at a distance of not less than 3/5 centimeters.
- The prick test can give false negative results if the patient is being treated with antihistaminesor corticosteroids , or in the presence of intercurrent pathologies, eg. AIDS or Hodgkin’s lymphoma .
- To increase the reliability of the prick test, it is essential to carry out a couple of pricks even on two control drops: the first based on saline solution and the second based on 1% histamine. Unlike the latter, the response to the saline solution must always be negative; conversely, the histamine response must always be positive. If this does not happen, for example due to poor or excessive skin sensitivity (often influenced by the aforementioned drugs), the prick test is not reliable and it is necessary to resort to other allergic investigations.
- The skin reactivity of the upper half of the back is higher than that of the lower half of the back, which in turn is higher than the forearm.
- Skin reactivity increases during pollination.
- In women, skin reactivity is greater during menstruation.
- Perform the prick test only on healthy skin.
- The lancet must be renewed with each puncture to avoid contamination between the different allergens. The same goes for the gauze strip used to remove the drop of allergen.
- The diagnostic accuracy of the prick test depends on the quality of the allergens used.
Are the prick test results reliable?
The prick test is a very sensitive test (> 90%), whose diagnostic value is above all negative: therefore, when faced with a negative result, allergic sensitization towards the allergen can be excluded. On the other hand, the prick test is burdened by a poor specificity and positive predictive power (50-85%): this means that a positive test is not able to ensure the actual presence of a patient’s hypersensitivity, so the diagnosis allergy final will always have to rely on other tests.
It is important to remember that we are talking about allergies, since the prick test is not able to highlight the presence of any food intolerances, but only reactions dependent on IgE .
Prick by Prick test
In cases of suspected food allergy , when faced with a negative prick test it is possible to resort to a very similar test, called prick by prick test. During this investigation, instead of using a diluted allergen extract (usually in glycerin), fresh food is used directly. Therefore, the pin is immersed in the food to be tested and subsequently made to penetrate the patient’s epidermis to carry out the prick test according to the procedures described above.
The advantage of the prick by prick test is that with this technique the limit of a possible poor stability of the allergenic solution is exceeded, and the consequent risk of false negatives . The prick prick test is therefore particularly useful in the presence of a discrepancy between the patient’s clinical history and the prick test results. Similarly to the latter, the prick by prick test maintains a low specificity, with the additional problem of non-standardization of the food source and the supply of fresh food.