Diagnostic Allergy testing

Skin allergy testing comprises a range of methods for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.

Certain medications can affect the reactivity of the skin and should be stopped before skin testing: antihistamines at least 5 to 7 days earlier, systemic corticosteroids at least 3 weeks if given for a long time, or at least 3 days after a short course, or at least 2 weeks if applied locally to the test site.

PRICK test (skin prick test)

PRICK test or skin prick test is most often performed for nutritional (food) and inhalation (pollen) allergens. Additionally, the test can be performed for preservatives and additives.

Skin prick test is used to detect the presence of specific IgE antibodies in the skin of patients who is hypersensitive to a particular allergen. The skin prick test is a reliable test for detecting a type I / antibody-mediated allergic reaction.

The test is performed in a sitting position. During testing, the allergen solution is applied in the form of drops on the skin (inner side of the forearm), and is inserted into the surface layer of the skin by pricking the lancet.

Both positive and negative controls need to be done for comparison. At the beginning of the test, testing for positive control (histamine) and negative control (buffer) is performed, which is read after 10 minutes. Positive control (histamine) allows to assess the reactivity of the skin and the strength of skin test, and negative control (saline) allows to exclude dermographism (reaction caused by a lancet sting) as the cause of a positive test. After 1 minute the solution is wiped off and the results are read after 20 minutes.

If hive develops at the injection site (elevated reddish or pale lesion in the skin of different shape and size), the test is considered positive. The most common are local reactions in the form of redness and itching of the skin.

There is a low risk of a systemic allergic reaction to the allergen, which may include urticaria, angioedema, rhinorrhea, bronchospasm, gastrointestinal symptoms (abdominal cramps, vomiting, diarrhea) and, ultimately, life-threatening anaphylaxis, which is why the test is performed under medical supervision. 

Intradermal test

Intradermal allergy testing is another method of skin testing to help determine whether an individual is allergic to a specific allergen. The test involves injection of a small amount of the suspected allergen under the surface of the skin. After about 20 minutes the area is examined for a reaction at the site.
A typical reaction looks like a small hive with swelling and redness. The intradermal test is more sensitive than the skin prick test and can usually provide more consistent results.

Intradermal test is a reliable test for proving a early-type allergic reaction. During this test, an allergen solution in an amount of 0.02 to 0.05 ml is injected into the skin using a needle. Both positive and negative controls need to be done for comparison. A positive control (histamine) allows the reactivity of the skin and the strength of the skin tests to be assessed, and a negative control (saline solution) allows the exclusion of dermographism (reaction caused by a needle prick) as the cause of a positive test. The results are read after 20 minutes. If hive develops at the injection site (elevated reddish or pale skin changes of various shapes and sizes), the test is considered positive.

The most common are local reactions in form of redness and itching and minor bleeding at the site of the needle puncture. There is a low risk of a systemic allergic reaction, which may include urticaria, angioedema, rhinorrhea, bronchospasm, gastrointestinal symptoms (abdominal cramps, vomiting, diarrhea) and, ultimately, life-threatening anaphylaxis. 

PATCH test 

PATCH or epicutaneous test is used to prove a late hypersensitivity reaction to contact allergens . It is a diagnostic method used to determine which specific substances cause allergic raction. 

Patch testing helps identify which substances may be causing a delayed-type allergic reaction in a patient, and may identify allergens not identified by blood testing or skin prick testing.

It is intended to produce a local allergic reaction on a small area of the patient’s back, where the diluted chemicals were planted.

The test is performed on the skin of the back where allergen patches are applied (standard set of 30 allergens). The test is read after 48 hours when the patches are removed, and additional readings are performed after 72 or 96 hours and an additional 7 days after setting up the patch test.

In the case of a positive reaction to a particular allergen appear redness, vesicles and edema which occurs at the site of contact with specific allergen.

Atopy patch test

Atopy patch test (APT) has been recognized as a useful diagnostic tool in the diagnosis of delayed type reaction to food allergen in children.

It has been reported as a diagnostic tool with high predictive capacity for late-phase clinical reactions in children with atopic dermatitis. Results are evaluated after 48 and 72 h.

APT is a useful diagnostic procedure in patients with atopic dermatitis allergic to inhalant allergens and with food allergy younger than 2 years. The sensitivity and specificity of the test greatly depend on the allergen tested and patient age.