Most physicians will begin a diagnosis of a possible penicillin allergy by examining a person’s medical history. This can help to quickly rule out penicillin as an allergen or suggest other conditions that might be involved. A physical examination will also help the physician determine whether penicillin is causing the problem. During the evaluation, the physician will look for signs of an allergic reaction, such as hives, a skin rash or angioedema (swelling of the lips, face or tongue).

There are two types of tests that are usually used to detect a penicillin allergy. These tests can also help identify whether an allergy is severe, or whether allergies to related substances (such as the cephalosporin family of antibiotics) are present. These tests include:

• Allergy skin test. This test involves scratching, pricking or injecting an individual’s skin with penicillin extracts (usually the penicillin components called major and minor determinants are used). The tested area will react with redness or swelling to indicate an allergic response. This test can be too dangerous to use on highly sensitive individuals.
  
  It should be noted that there is no licensed penicillin skin test in the United States. Physicians who perform skin testing to penicillin make up their own skin testing materials, and this may not be as accurate as a standardized test.  

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• RAST (radioallergosorbent test). This type of blood test allows a laboratory to directly test a blood sample from an individual in an attempt to detect antibodies that correspond to a penicillin allergy. Although rarely performed in cases of drug allergies, this test can be used on people who are too sensitive for a skin test.