20 February 2020 What types of skin tests are there? There are several types of skin tests , namely: Prick tests : they are made with commercial extracts, they are painless, fast to interpret and extremely safe. Prick-prick tests : used to assess food allergy, a prick is made on the food in question and then on the patient skin. Intradermal tests : used to assess drug allergy, they consist of the intradermal injection (into the skin) of a small amount of medicines. Patch tests : consist of exposing the skin on the back to suspected allergens considering the patient clinical history, and evaluate the results at 48 and 72 hours, keeping the allergens in contact with the skin during this time period. Skin tests are usually performed on the forearm , except for epicutaneous tests, which are performed on the back . It is essential that skin tests are performed by professionals with adequate training , to ensure their safety, reproducibility and clinical relevance. Why should skin allergy tests be done? Each allergic disease needs specific investigation. For example, skin prick tests, the most frequently performed, are essential in suspected respiratory allergy. Prick-prick tests are important in food allergy, intradermal tests in drug allergy and epicutaneous tests in contact dermatitis. How are skin prick tests and prick-prick tests performed? Prick tests and prick-prick tests are the first diagnostic test when an allergy is suspected. Its simplicity , speed of execution, low cost and high sensitivity , justify this choice. Through a small bite , a small amount of allergen (antigen) is introduced into the epidermis. If the patient has immunoglobulin (Ig) E specific for that allergen , then a reaction develops , which appears as a papule, itching and erythema after exposure to the antigen, peaking at 20 minutes (immediate reaction). A drop of commercial allergen extract is placed on the surface of the skin (arm) that is lifted with a small lancet. Positive (with known reaction) and negative (non-reaction) controls are also placed. At 15-20 minutes, papules are recorded. When there are no commercial allergens , as in the case of some foods , the prick-prick technique is used, in which the skin is bited after the lancet bites the suspect food. How patch tests are performed? Epicutaneous or patch tests are preferably indicated in the study of allergic contact dermatitis , in eczemas with precise locations (for example, hands, periorbital and others) and also in eczemas resistant to treatment . Other clinical situations may require a complementary study with epicutaneous tests, such as non-immediate drug allergy. Allergens are grouped into different categories , available for evaluation by epicutaneous tests. These can be based, among others, on: work / leisure activity (eg, hairdresser or baker series); location of dermatitis (eg, shoe series); similar chemical structure between compounds (eg, acrylates or epoxy resins); functional relationship (eg, corticosteroids series or cosmetics series). In epicutaneous tests, a small amount of a series of suspicious substances is placed on the skin of the back with subsequent occlusion with an adhesive dressing . The dressings should be removed after 48 hours and the readings are taken subsequently depending on reactions and the allergens tested, 72 and/or 96 hours after placement. For some series an additional reading after 7 days is recommended to observe any very late reactions. The authors of this text are the following allergologists from the Hospital da Luz network: Luís Miguel Borrego, Ana Morais Mendes, Ana Reis Ferreira, Andreia Ferrão, Carlos Braga, Cristina Lopes, Diana Pereira da Silva, Filipa Semedo, Isabel Costa Silva, João Gaspar Marques, Pedro Carreiro Martins, Marta Salgado, Nuno Sousa, Rosa Anita Fernandes, Susana Piedade The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
