Allergy Tests

General Description

Allergy tests are a diagnostic technique used to determine which substances, or allergens, trigger an allergic reaction in a patient.

Allergies occur when the immune system overreacts upon contact with certain substances, perceiving them as harmful and triggering an inflammatory response in different parts of the body, such as the skin, eyes, digestive tract, or respiratory system. Allergens can enter the body through skin contact, inhalation, ingestion, or injection. Common allergens include pollen, dust mites, pet dander, latex, insect stings, and certain foods and medications.

When Are They Indicated?

Allergy tests are recommended when a patient presents symptoms characteristic of an allergic reaction:

• Ocular symptoms: itching, redness, and tearing.
• Nasal symptoms: congestion, runny nose, itching, sneezing.
• Respiratory tract symptoms: coughing, difficulty breathing, wheezing.
• Skin symptoms: dryness, itching, rashes.
• Digestive tract symptoms: abdominal pain, bloating, diarrhea.
• Systemic symptoms: anaphylactic reaction.

How Are They Performed?

Allergy tests are based on controlled exposure of the body to potential allergens to confirm an allergic reaction. To do this, allergenic extracts are used—aqueous solutions containing biomolecules derived from natural sources of allergens.

Depending on the specific method, allergy tests are classified into different types:

Skin Allergy Tests:

These involve introducing the allergenic extract into the skin. Different techniques include:

• Skin prick tests (prick-test): A few drops of the extract are applied to the skin, and a series of pricks or scratches are made with a lancet to allow the extract to enter the skin.
• Intradermal tests: The extract is injected directly into the dermis.
• Patch tests (epicutaneous tests): Allergenic extracts are placed on a special patch that remains adhered to the skin for 48 to 72 hours.

Blood Allergy Tests:

A blood sample is drawn to measure specific Immunoglobulin E (IgE) levels—an antibody produced by the body in response to specific allergens. Levels are measured by adding the specific allergen to the blood sample and observing the formation of allergen-antibody complexes.

Exposure Allergy Tests:

Used for possible food or medication allergies, these involve ingesting the substances in measured doses, starting with very small amounts and increasing them until a reaction occurs. In some cases, the substances may be administered intravenously instead of orally.

Provocation Allergy Tests:

Allergens are applied directly to the affected mucous membranes, such as the eyes, nose, or lungs. As with exposure tests, allergen doses are administered in a controlled and increasing manner until symptoms appear.

In some cases, multiple types of allergy tests may be necessary to reach a definitive diagnosis, typically starting with skin and blood tests. If these results are inconclusive, exposure or provocation tests are performed.

Risks

The primary risk of an allergy test—especially exposure and provocation tests—is the possibility of an anaphylactic reaction, a medical emergency that can be life-threatening. However, this is a rare occurrence, as tests are conducted in controlled environments with low doses of allergens.

What to Expect from an Allergy Test

Skin or patch tests are usually performed on the skin of the arm or, in the case of patch tests, on the back. The patient must remove any clothing covering these areas, which the doctor cleans and disinfects before beginning the procedure. Multiple allergens are applied at the same time. The pricking or injection may cause mild discomfort but is not particularly painful.

For skin prick or intradermal tests, the skin is examined 15 to 20 minutes after exposure. If a rash appears, the allergy test is considered positive. The total duration of the procedure depends on the number of substances being tested but is usually less than an hour.

For patch tests, the patch is removed, and an initial assessment is done at 48 hours, with a second evaluation after more than 72 hours. While wearing the patches, patients must keep them dry while bathing and avoid activities that cause excessive sweating, as they could shift or detach.

For an IgE blood test, the sample is collected via venipuncture with a needle, usually from the forearm.

Both skin tests and blood sampling are outpatient procedures, allowing patients to return home immediately afterward.

Exposure and provocation tests, however, are performed in a hospital setting to ensure immediate medical intervention in case of anaphylaxis. The allergenic substance is administered gradually, with observation periods between doses, so the study may last several hours.

Specialties That Request Allergy Tests

Various allergy tests are conducted within the allergology unit.