We have evaluated over 4,000 patients with a history of reactions to penicillin(s) (penicillin, amoxicillin, or cephalosporins). After evaluation and testing, we have found that approximately 90% of these patients can safely take these drugs without fear of a life-threatening reaction. Realizing that these patients had reactions prior to being seen, how does one explain this apparent discrepancy? There are several possibilities:
- We do know that truly allergic people, with time, can lose their sensitivity to penicillin.
- An infectious agent (such as viruses or bacteria) and not necessarily the antibiotic could have caused the initial reaction.
- If amoxicillin is taken when a patient is infected with certain viruses, such as infectious mononucleosis, a rash can develop.
- Amoxicillin can cause a non-itchy measles like rash in 7-10% of all people who take it. The majority of people who develop this reaction can take penicillin(s) again without a life threatening reaction or even a recurrent rash.
- In some cases there is no explanation.
To prove a penicillin allergy exists, skin testing with penicillin and one of its breakdown products are used. You will be tested with what we call the major breakdown product of penicillin (PrePen), plain penicillin, and in some patients, ampicillin or cephalosporin. In addition, skin testing with salt water (to which no one should react) and histamine (to which everyone should react) is used to help distinguish between negative and positive skin test reactions. Depending on the patient's history and age, testing usually will consist of these reagents being placed on the forearm and then scratched with a little scratcher (small plastic disposable peg). A positive reaction will look like a mosquito bite at the site of the scratch. Ten to fifteen minutes later, if this shows no reaction, a small amount of each reagent (five to six) will be injected under the skin with a small sterile needle and observed for fifteen minutes for a "mosquito bite" like reaction. This is called intradermal testing. If after fifteen minutes the intradermal skin tests show no reaction, some people will then immediately be given a penicillin or related antibiotic in the office and observed for sixty minutes. The oral penicillin is either given as a single dose or in divided dosages. Others might be asked to observe the area where intradermal tests were placed for up to 72 hours, looking for a "mosquito bite" type reaction, before receiving the oral penicillin or related antibiotic. If, after an hour, you have had no reaction to penicillin, you will then be sent home and may be asked to take further dosages of the antibiotic tested and / or a related antibiotic, observing for the development of any rash. The chances of a rash occurring are rare. The rashes are usually mild and self-limiting. If a rash occurs, stop the antibiotic and call the office. The history, skin test, and oral challenge will take approximately two hours. Depending on the history and the age of the patient, some people will be brought back four to six weeks later to be re-skin tested to see if perhaps you might have been resensitized by taking the penicillin. This will take approximately twenty minutes. We have found that this occurs only in a very small number of patients. However, because of this possibility, if the physician feels the skin tests need to be repeated, it is very important for it to be done before we can say that you can safely take penicillin in the future.
What we do is called elective testing. We feel that the best time to test a child or an adult is when they are well and not in immediate need of this antibiotic. It may not be practical to test when a patient is sick. We have found that the skin testing is safe, and following a negative skin test, no one has ever suffered a severe life threatening reaction when given the penicillin. We feel that if your penicillin evaluation is negative, it is safe for you to take penicillin in the future, because your chance of experiencing a reaction is no greater than it would be for a person without a history of penicillin allergy.