Small amount of drug is injected in the skin and if weal and flare are formed allergy to that drug is suspected. It is very accurate for drugs such as penicillin but for other drugs it gives only a crude idea.
Skin tests. With a skin test, the allergist or nurse administers a small amount of a suspect drug to your skin either with a tiny needle that scratches the skin, an injection or a patch. A positive reaction to a test will cause a red, itchy, raised bump. A positive result suggests you may have a drug allergy.