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Drug Allergy Testing, including Penicillin Allergy Testing
Skin testing for penicillin allergy is the most reliable way to determine if a person is truly allergic to penicillin. Approximately ninety percent of people will test negative (meaning they do NOT have a penicillin allergy), because they either lost the allergy over time, or they were never allergic in the first place. There are some situations in which penicillin, which is generally safe and inexpensive, would be a suitable antibiotic, but a person with possible allergy is given a stronger drug with more side effects because their allergic status is unclear. Therefore, determining if someone can safely take penicillin can be useful.
Testing for penicillin allergy is especially important in the following situations:
People who have a suspected penicillin (or closely related antibiotic) allergy and require penicillin to treat a life-threatening condition for which no alternate antibiotic is appropriate.
People who have frequent infections and have suspected allergies to many antibiotics, leaving few options for treatment.
Penicillin skin testing does NOT provide any information about certain types of reactions. This includes severe reactions with extensive blistering and peeling of the skin (Stevens-Johnson syndrome or toxic epidermal necrolysis), a widespread sunburn-like reaction that later peeled (erythroderma) or a rash composed of small bulls-eyes or target-like spots (erythema multiforme). People with these types of reactions should never again be given the medication that caused the reaction. This applies to all situations since a second exposure could cause a severe progressive reaction and even death.
Skin testing should be done by an allergist in an office or hospital setting. Testing usually takes about one hour to complete. The skin is pricked and injected with weak solutions of the various preparations of penicillin and observed for a reaction. This may cause discomfort due to itching, although it is not painful.
A positive skin reaction is an itchy, red bump that lasts about half an hour and then resolves. A positive test indicates that the person is truly allergic. People with a positive test should continue to avoid penicillins.
If the patient completes the skin testing without a positive reaction, a single oral dose of full strength penicillin [amoxicillin] is commonly given to confirm that the patient does not have an allergy to the medication. [This amoxicillin “challenge” is typically performed right after the penicillin testing, while still under observation. ] The oral dose is needed because medical tests, including skin testing, are rarely 100 percent accurate. About three percent or less of people with a history of penicillin allergy and a negative skin test will still experience an allergic reaction [to this amoxicillin “challenge”] . However, these reactions are [usually] very mild. If a person has a negative skin test and has no reaction to an oral dose of the antibiotic [the amoxicillin “challenge”], no future precautions are necessary [with regards to penicillin.]
Adapted from UPTODATE Copyright 2013. Items in  added by Bernard Feigenbaum, MD
FOR MORE ON PENICILLIN ALLERGY TESTING
FROM THE CDC: