What are the symptoms of skin test allergy?

Because everyone's physique is different, the medicines used are also different. Some medicines will cause allergic reactions in some people, so before using the medicine, the doctor will do a skin test to see if there is an allergic reaction. If there is an allergic reaction, different symptoms such as redness, swelling, and lumps will appear.

1. The results of type I hypersensitivity reactions are observed within 20 to 30 minutes after antigen stimulation. The positive reaction of the prick test is mainly redness, while the positive reaction of the intradermal test is mainly wheal. The judgment criteria are shown in Table 24-2.

2. The results of type IV allergic reaction are observed within 48 to 72 hours after exposure to the antigen. The positive results of intradermal tests are mainly redness, swelling and nodules, while the positive results of patch tests are mainly redness, swelling and blisters. The judgment criteria are shown in Table 24-3.

3. False results Under certain conditions, the results of skin reactions may not match the actual conditions of the body, that is, false positives or false negatives may occur.

Common reasons for false negatives include:

① The concentration of the test antigen is too low or it fails for various reasons;

② The patient is taking immunosuppressants or antihistamines during the test (the latter can be determined by setting up a histamine positive control;

③ Operational errors, such as injecting too deep into the subcutaneous tissue during intradermal testing, injecting too little antigen, etc.;

④ Improper selection of skin test season. For example, after the pollen season, the level of anti-pollen antibodies may decrease.

Common causes of false positives include:

① The test antigen is impure and is contaminated by other antigens during extraction, preparation, or even during the test, causing cross-reactions;

② Improper preparation of the test solution, excessive acidity or alkalinity will cause non-specific irritation to the skin;

③ Excessive skin reactivity, for example, the subject suffers from skin scratches or has traces of previous allergies, etc.;

④ Improper operation, such as injecting a small amount of air, may also cause a false positive.