Allergy Test Results Scale or prick tests are performed on the skin in order to identify the substance that can trigger an allergic reaction in the patient. Know how they are carried out.
The results of the allergy tests or prick tests are read 15 minutes after the puncture, evaluating the size of the possible papules and the reddened area (erythema) that may have formed.
There are several ways to graduate the result, and one of them is the following:
- When there is no papule or erythema, or there is an erythema less than 1 mm, it is classified as -.
- The absence of papule or the presence of a very light papule together with an erythema no greater than 3 mm in diameter, is classified as +.
- A papule of up to 3mm or erythema no greater than 5mm in diameter is classified as ++.
- A papule between 3 and 5 mm in diameter with erythema is classified as +++.
Any major reaction is classified as ++++.
There are some errors in the performance of the test that can falsify the results, such as:
- Put the extracts together (less than 2 cm apart) so that the reactions come together and prevent their visual separation.
- Perform a puncture that penetrates insufficiently into the skin. This results in a false negative result, that is, a test that is actually positive is negative.
- Use too much of the substance to study, so that it can spread and mix with the other extracts.
- Produce bleeding with the puncture. This can lead to a false positive result, that is, the substance produces an irritation that is diagnosed as an allergy, when in fact there is no allergy to the allergen.
- Having taken antihistamines prior to the tests. This can result in a false negative result.
- Propagation of allergens at the time of testing or when removing the allergenic solution. It can lead to a false positive result.
Results of the patch test
The results of the patch tests are read 48 hours after patch placement. The classification can be as follows.
- When there is no reaction it is classified as -.
- When there is a pale erythema or a doubtful reaction, it is classified as?
- When there is a mild erythema it is classified as +.
- The presence of a papule with erythema, edema and vesicles is classified as ++.
- The presence of a papule with edema, large or grouped vesicles, or a very strong reaction, is classified as +++.
A false negative result can also be given when the patch is misplaced, the reaction is read within 48 hours, or a medication has been taken that decreases allergic reactions, such as corticosteroids. On the contrary, there are several circumstances that can give rise to a false positive, for example if the patch is placed on an already irritated area or a reaction occurs, not to the allergen tested, but to the adhesive tape of the patch.
The doctor will give you the results by means of a written report in a few days. The report of the result will include data on the technique used, the allergens tested and the reaction obtained with each one of them. The doctor will indicate the need to do other complementary scans or some treatment.