Penicillin Allergy Case Discussion

Case
A 26-year-old woman who has mitral stenosis requires extensive dental surgery. Penicillin prophylaxis against streptococci is indicated, but the patient is allergic to penicillin. She states that 15 years ago she had hives and wheezing 30 minutes after she had taken oral penicillin.
How would you determine whether the patient is likely to have an allergic reaction if she is treated with penicillin now?
What do you do if the skin test result to penicillin is positive?
What is the prevalence of allergic cross-sensitivity between penicillin and cephalosporins?
If the patient's skin test result to penicillin proves to be negative, how certain is it that she is not allergic?

If the skin test result to penicillin is positive, could you avoid a reaction by giving penicillin orally instead of by injection?
If penicillin must be used because there is no acceptable alternative, can this patient be rapidly desensitized?

Case Discussion
How would you determine whether the patient is likely to have an allergic reaction if she is treated with penicillin now?
Skin testing for penicillin can be an extremely useful procedure for determining whether a patient, who has a history of penicillin allergy and in whom an IgE-mediated immunologic mechanism is suspected, is likely to have an allergic reaction to a later exposure to penicillin. If all of the reagents are available, the reliability of these tests has been as high as 96% in studies of patients who had a history of allergy, whose skin test results were negative, and who were subsequently challenged with penicillin. The testing should be done by a person familiar with the procedure. The reagents used include histamine (the positive control), saline (the negative control), penicilloyl polylysine (Pre-Pen), the minor determinant mix (MDM), and the penicillin that will be used for treatment. For the test result to be positive, the patient must show a positive reaction to histamine, a negative reaction to saline, and a positive reaction to Pre-Pen, MDM, and/or the native penicillin. The positive reaction consists of a wheal and flare that appears in 15 minutes. Unfortunately, because of the rare need for this test and extreme caution by the U.S. Food and Drug Administration (FDA), neither Pre-Pen nor the MDM is available. For this reason, we almost always use alternative drugs. If the history is not suggestive of a type I immediate hypersensitivity reaction and no alternative drugs are satisfactory, a test dose is given under controlled conditions. If the history is suggestive of a type I reaction and no alternative drug is available, we can desensitize the patient (see question 6 in following text).
What do you do if the skin test result to penicillin is positive?
First, you always look for an effective nonpenicillin drug substitute and use it. If one is not found, consider desensitizing the patient to penicillin.
What is the prevalence of allergic cross-sensitivity between penicillin and cephalosporins?
The cephalosporins resemble the penicillins chemically, but the true prevalence of cross-reactivity between semisynthetic penicillins and cephalosporins is not known because investigators cite discordant results. A reasonable estimate is that 5% of penicillin-allergic patients are sensitive to third-generation cephalosporins.
If the patient's skin test result to penicillin proves to be negative, how certain is it that she is not allergic?
Assuming that the entire panel of skin tests (including the controls) were done properly and the results interpreted correctly, a negative skin test result is a reliable
indicator that an acute IgE-mediated reaction will not occur. However, the skin test has no bearing on IgG-mediated reactions.
If the skin test result to penicillin is positive, could you avoid a reaction by giving penicillin orally instead of by injection?
No. Oral penicillin can also sensitize and elicit an acute reaction in already sensitized individuals.
If penicillin must be used because there is no acceptable alternative, can this patient be rapidly desensitized?
Yes. However, this is a potentially dangerous and always time-consuming procedure. It should be done in the intensive care unit by an experienced allergist using published protocols.