Sometimes scratch testing, also called prick testing, is confused with intradermal testing. The fact is, they are vastly different.
What is prick testing?
Prick testing is done by placing a small amount of an allergen on the skin, and then scratching or pricking the surface so the antigen seeps into the skin. The reaction is then measured and a judgment is made by the doctor or tester. There is a high chance of false positives with this method, meaning it will say the patient is allergic to something when they really aren’t.
What is intradermal testing?
Intradermal testing means the allergen is placed directly under the skin using a needle. A small bump appears under the skin and is measured after the injection, and then again 5-10 minutes later. If the bump increases in size, the patient is likely allergic to the antigen. With intradermal testing, different dilutions are usually used. The tester will probably start with a concentrated dose of the allergen, meaning it’s higher in strength, and then the allergen is diluted (gets less strong) until the bump stops growing is size. This is the patient’s end point, and the starting point for treatment.
So, what does all that mean?
In short, intradermal testing is under the skin, while prick testing is above the skin and then scratched. Intradermal testing will usually use different dilutions, or strengths, which can give a more accurate representation of the level of allergic reaction from the patient. In addition, intradermal testing is usually only available from specialized doctors, while prick testing is usually done by most primary care doctors.