D. Skin prick testing should be read after 48 hours – Explanation
Skin prick testing can be read after 15-20 minutes. Skin patch testing is read after 48 hours
Allergy tests
| Skin prick test | Most commonly used test as it is easy to perform and inexpensive. Drops of diluted allergen are placed on the skin, which is then pierced using a needle. A large number of allergens can be tested in one session, typically including a histamine (positive) and sterile water (negative) control. A wheal will develop if the patient has an allergy. Results can be interpreted after 15 minutes. Useful for food allergies and pollen allergies. |
| Radioallergosorbent test (RAST) | Determines the amount of IgE that reacts specifically with suspected or known allergens, such as IgE to egg protein. Results are graded from 0 (negative) to 6 (strongly positive). Useful for food allergies, inhaled allergens (e.g., pollen), and wasp/bee venom. Blood tests like this are useful when skin prick tests aren’t suitable, such as in cases of extensive eczema or antihistamine use. |
| Skin patch testing | Used for contact dermatitis. Around 30-40 allergens are placed on the back, and irritants may also be tested. The patches are removed after 48 hours, and results are interpreted by a dermatologist 48 hours later. |
