Intradermo test

What is the intradermal test?

The intradermal test or intradermal test is one of the techniques carried out to detect substances to which the patient shows an allergic reaction. It is a skin test that provides a reliable diagnosis. The objective is to try to reproduce an allergic reaction on the skin and test the substance to which the patient is allergic. Allergies occur when the allergen enters the body and the immune system responds with many antibodies called immunoglobulin E (lgE).

Immunoglobulin in turn is related to mast cells which, when they come into contact with allergens, release chemical mediators, including histamine, which are responsible for producing the typical symptoms of allergic reactions.

The intradermal test makes it possible to observe the reactions to allergens
by injecting them into the skin surface

What does it consist of?

The intradermal test consists of injecting small amounts of the substances or allergens under the patient’s skin. Disposable syringes with tuberculin, 1ml and 26 gauge, are used for this purpose. The test area is marked and cleaned with alcohol. Keeping the skin taut, the allergist inserts the needle parallel to the skin surface, as superficially as possible.

As this is a very sensitive procedure, the antigen solutions must be more diluted than in the prick test. It is a test that should only be performed if, previously, the results of the prick test are negative or doubtful, since if it is performed on highly sensitized patients it can cause anaphylactic reactions.

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In addition, saline will be used as a negative control method to rule out false positives, since the intradermal test is a more sensitive test but not as specific as the prick test or others.

Why is it performed?

It is a test that is usually performed when the prick test is negative but it is still suspected that the patient may be allergic to certain substances. The results will be observed in a period of time between 10 and 30 minutes. When the test is performed, it will be positive with the appearance of a wheal of about 5mm and erythema of about 10mm.

Preparation for the intradermal test

Before undergoing the intradermal test the patient should stop taking antihistamines. It is usually sufficient to stop them three days before the test, but with some patients a longer interval of days will be necessary.

How do you feel during the test?

During the intradermal test the patient will notice the allergic reaction to the substance in question. However, it should always be controlled by the specialist in Allergology, who will assess the clear symptoms of allergic reaction with mild hives, swelling and redness, with the formation of a wheal of between 5 and 10mm. In addition, to avoid delayed reactions, a second reading of the test should be done after 6 hours.

Meaning of abnormal results

The specialist should analyze how the reaction to the injected substance is progressing. Thus, if he sees any exaggerated skin rash or a possible anaphylactic reaction, he should react immediately, in order to stop as soon as possible the symptoms that the allergen is causing in the patient.