Tests and diagnosis

By Mayo Clinic Staff

A thorough exam and appropriate diagnostic tests are essential for an accurate diagnosis. Research has shown that penicillin allergies may be overdiagnosed and that patients may report a penicillin allergy that has never been confirmed. A misdiagnosed penicillin allergy may result in the use of less appropriate or more expensive antibiotics.

Your doctor will conduct a physical examination and ask you questions. Details about the onset of symptoms, the time you took medications, and improvement or worsening of symptoms are important clues for helping your doctor make a diagnosis.

Your doctor may order additional tests or refer you to an allergy specialist (allergist) for tests. These may include the following.

Skin tests

With a skin test, the allergist or nurse administers a small amount of the suspect penicillin to your skin either with a tiny needle that scratches the skin or an injection. A positive reaction to a test will cause a red, itchy, raised bump.

A positive result indicates a high likelihood of penicillin allergy. A negative test result usually means you're not allergic to penicillin, but a negative result is more difficult to interpret because some kinds of drug reactions cannot be detected by skin tests.

Graded challenge

If the diagnosis of a penicillin allergy is uncertain or your doctor judges an allergy unlikely based on the symptoms and test results, he or she may recommend a graded drug challenge.

With this procedure, you receive four to five doses of the suspect penicillin, starting with a small dose and increasing to the desired dose. If you reach the therapeutic dose with no reaction, then your doctor will conclude you aren't allergic to that type of penicillin. You will be able to take the drug as prescribed.

Similarly, if you are allergic to one type of penicillin, your doctor may recommend a graded challenge with a type of penicillin or cephalosporin that's less likely — because of known chemical properties — to cause an allergic reaction. This would enable your doctor to identify an antibiotic that can be used safely for a current bacterial infection and guide choices in future treatments.

During a drug challenge, your doctor provides careful supervision, and supportive care services are available to treat an adverse reaction.

Nov. 22, 2014

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