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Make the Diagnosis: Penicillin Allergy

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Prior Probability

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About 10% of patients will have an adverse reaction to penicillin, but most will not be penicillin allergic. Less than 1% of all patients will have a true allergy to penicillin, as defined by an anaphylactic reaction, positive skin test result, or response to a second dose of penicillin. Physicians should ascertain the nature of the reaction to help decide whether it might have represented a true penicillin allergy. An allergy to penicillin may be difficult to ascertain from the patient's medical history, primarily because many penicillin reactions do not represent allergic reactions. The most important finding from a penicillin history is also the least frequent—patients with severe reactions (eg, toxic epidermal necrolysis, life-threatening anaphylaxis, hemolytic anemia, liver damage) should not be skin tested for penicillin allergy and should not receive penicillin.5

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Population for Whom Penicillin Allergy Should Be Considered

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  • Patients with a previous allergic response to penicillin-type antibiotics

  • Patients with a response to medications that cross-react with penicillin (eg, cephalosporins, carbapenems)

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Detecting the Likelihood of Penicillin Allergy

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The history of a penicillin allergy or a negative skin test result affects the probability of a true penicillin allergy (Table 39-4).

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Table Graphic Jump Location
Table 39-4Likelihood Ratios and Predictive Value for a History of Penicillin Allergy in Predicting True Allergy
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Reference Standard Tests

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Penicillin allergy is confirmed by a reliable history of an immediate anaphylactic reaction, positive skin test reactivity, or well-documented response to a second observed penicillin challenge.

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Original Article: Is This Patient Allergic to Penicillin?

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Clinical Scenarios

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Case 1
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An 18-year-old male college student presents with group A streptococcal pharyngitis, and you prescribe penicillin.1 The patient informs you that he developed a rash after taking about half a penicillin prescription for a respiratory tract infection 3 years ago. The rash was bright red, was restricted to the extremities and trunk, and resolved several days after penicillin was discontinued.

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Case 2
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A 26-year-old pregnant woman has syphilis. She recalls an “itchy ...

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