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Prick test: allergy skin tests | Diseases and conditions: Medical tests.


What is the prick test or allergy tests

Skin prick test or allergy tests are tests which are made on the skin in order to identify the substance that can trigger an allergic reaction in the patient. This is inserted into the skin a tiny amount of the possible causative agent of allergy. We call this substance that triggers an allergy allergen. When the allergen triggers a reaction in the skin is considered that the individual is allergic to that substance.
In general there are two systems for diagnosis of allergic diseases. On the one hand they are laboratory methods measuring different substances originating from the allergen, as for example a few antibodies called IgE that are specific to the different substances that cause the problem (mites of dust, milk, fish, etc) in the blood. On the other hand are tests in vivo, of which there are three variants:
Immediate hypersensitivity skin tests, also known as skin puncture or prick test tests.
Delayed hypersensitivity allergic tests, also called patch tests, patch or patch test testing.
Provocation tests, which consist of managing allergen controlled by nasal, bronchial, conjunctival, orally or parenterally. The most common is the bronchial provocation test for the diagnosis of some respiratory allergen-induced asthma.
In this chapter we will focus us in allergy skin tests known as the prick test or patch test.
Of puncture or prick test skin tests are considered the method most appropriate for the diagnosis of allergy to inhalants for their safety and reliability. It consists of the application of different allergens in the forearm or back. The intensity of the reaction allows you to discover the cause of the allergy and the magnitude of it. Allows us to study a possible allergy to drugs (such as antibiotics, anesthetics...), to pollens, epithelia of cat and dog, mites, fungi, plant and animal foods, latex, insect (mosquito, cockroach), fabrics (cotton), and other many substances.
The patch testing or the patch tests are tests that are performed to identify mainly substances which cause of contact allergy or allergic dermatitis. It consists of a small disc which contains the allergen and that sticks to the back of the patient. They are useful for example in eczema by make-up, or allergies by jewelry or different metals.

Allergy test prep

These are the issues you should keep in mind when you go to a test of allergy or prick test:
Duration: the test of the prick test is performed in a few minutes. The possible skin reaction is examined after 15 minutes. The patch test is also performed in a few minutes, but the skin reaction is examined at 48 hours, so you have to go twice to the query.
Entry: the test can be done on an outpatient basis, but I should occur a reaction very serious, which is exceptional, the patient can be entered.
Is necessary to be accompanied?: do not need to be accompanied.
Drugs: is not necessary to take any prior medication. It is very important to not take antihistamines or corticosteroids prior to the test, as they may reduce allergic reactions and distort the results. The doctor will tell you with how long in advance should discontinue this medication.
Food: in general, it is necessary to go on fasting.
Clothing: you can wear any clothes you want, but it is important that you can easily discover the arms or shoulders, depending on where you place the allergen.
Documents: should be informed consent, which is a role where they explain you what is the test and its potential risks. You should read it carefully and take it signed. You must take the wheel of the test request.
Contraindications: It should not be to test for allergies in the following cases:
  • Taking medications such as steroids or antihistamines that can interfere with the results.
  • Skin lesions that hinder the interpretation of results.
  • Severe or unstable patients that may worsen when the test gives an important adverse reaction.
  • The pregnancy.
Pregnancy and lactation: these tests should be avoided during pregnancy because of the risk of serious reactions that may damage the fetus from occurring. In addition, immunity is altered in pregnancy and the results may not be conclusive. However, it is generally considered that allergy tests can be made during breastfeeding, as the substances used are not going to reach the infant.

Other considerations

It is very important to not take antihistamines or corticosteroids prior to the test, as they may reduce allergic reactions and distort the results.

Allergy test is performed

Allergy tests are usually made in a service of Allergology. The prick test technique is done in a single visit to the query. Initially inform you about the technique that will be performed and possible skin reactions which may appear as a small papule or itching at the puncture points.
Cleaned you with alcohol the area chosen for the test, which can be the inner side of the forearm or back. Once evaporated alcohol, you will receive on the skin a drop with a summary of the substance being studied. Usually different extracts, are tested by what are several separate droplets about 2 cm each other. The technician who carried out the test will catch a small Lancet and with it cross the drop at a 45 ° angle. It slightly prick skin doing a very superficial puncture and trying to not make blood. Prick rise minimally skin to put there a small fraction of the allergen. Used a different Lancet for each extract to study. Then puncture with a small adjacent signal points are marked and the remains of the droplets are cleaned.
They will tell you to wait between 10 and 20 minutes, usually 15 minutes, to proceed to read the results of the test, i.e., to examine the possible skin reaction that may have occurred. It is very important that don't scratch the test area during this time to not mask results. The reactions that may occur after that time, when the patient has already abandoned the inquiry, are not taken into account.

How is the test of the patch or patch test

To carry out the test of the patch or patch test must go twice to the query. We will inform you the technique that you will perform. You will be back a few small discs impregnated with a small amount of the substance or substances will be assessed. These small discs are placed in two rows and remain 48hrs. Passes those hours must return to the query to read the results, i.e., to evaluate the skin reaction that it has been able to produce.
During these 48 hours need to follow some tips to not alter the results: it should not get wet back, avoid the heat, sweating, and sudden exercises that can remove tape, no scratching or expose to light the area, not to take drugs that distort the results such as corticosteroids or antihistamines. Finally, if patches begin to fall off, we will fix them with a small adhesive bandage to prevent possible reactions to the tape that distort the results.
Skin allergy techniques are very safe testing. In very rare cases can occur potentially serious reactions with the prick test in patients especially sensitive to any allergen. Allergy symptoms can range from a feeling of heat and the appearance of a skin rash, to a very severe allergic reaction called anaphylactic shock. There will always be a team ready to solve these rare eventualities. Allergy is often immediately after administration of the allergen. However, very rarely, you may receive allergy hours after contrast administration, causing itching, difficulty swallowing or breathing. In such a case you should urgently contact a doctor.

Allergy test results

Tests for allergy or prick test results are read within 15 minutes of puncture evaluating the size of possible papules and reddened area (erythema) which have been formed.
There are several ways to adjust the result, and one of them is as follows:
When there is no erythema or papule, or there are less than 1 mm Erythema is classified as.
The absence of papule or the presence of a very light papule with one rash of no more than 3 mm in diameter, is classified as a +.
A papule of up to 3mm or Erythema not exceeding 5 mm in diameter is classified as ++.
A papule 3 to 5 mm in diameter with Erythema is classified as a +++.
Any major reaction is classified as a +++.
There are some errors in the test which can distort the results, for example:
Put too much together extracts (less than 2 cm apart) so that the reactions together and prevent their visual separation.
Make a puncture which penetrates form under the skin. This originates a false-negative result, i.e., a test that is actually positive gives negative.
Use too much of the substance to study, so that it can spread and mix with other extracts.
Produce bleeding puncture. This can result in a false positive result, i.e. the substance produces an irritation that is diagnosed as allergies, when in fact there is no allergy to the allergen.
Have taken antihistamines prior to the testing. This can result in a false-negative result.
Propagation of allergens in the time of the test or to remove the allergen solution. It can result in a false positive result.

The patch test results

Patch tests results are read 48 hours after the placement of the patch. The classification may be in the following way.
When there is no reaction is classified as.
When there is a pale erythema or doubtful reaction is classified as?
When there is a slight Erythema 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 +++.
Also a false-negative result can be when placed bad patch, read the reaction within 48 hours, or taken any medication that reduces allergic reactions, such as corticosteroids. On the other hand, there are various circumstances that can lead to a false positive for example if it is placed the patch on an already irritated area or if there is a reaction, not the allergen tested, but the patch adhesive tape.
The doctor will give you the results via a report written in a few days. The result report will include data on the technique used, tested allergens and the reaction obtained with each of them. The doctor will indicate the need for any treatment or other complementary explorations.
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