When was the last time you thought about penicillin allergy? It seems this very common clinical issue comes up every day in the medical world but never seems to have a final answer. I find that every few years, there’s a new clinical update to “clarify” the question. So, toward the end of “clarifying” the issue of penicillin allergy a bit more, here’s our public service announcement to keep us all informed.

A Quick Review: Types of Hypersensitivity Reactions

Before we go too far down the antibiotic road, here’s a rapid review of the major types of hypersensitivity reactions. You’ll recall that penicillin allergy is a type 1 IgE-mediated hypersensitivity reaction. This is the type we’ll focus on here.

The Gell-Coombs classification⁶:

Ok. Let’s get to it…penicillin allergy, here we come!

First some important statistics for perspective:

• 1% of the population has an IgE-mediated immune reaction to penicillin (ie, a true allergy).¹
• 80% of patients with an IgE-mediated allergy lose it by 10 years after initial exposure.¹

What are patients actually reacting to when they have a penicillin allergy?

Reactive intermediates (termed antigen determinants)

Determining true PCN allergy:

Signs consistent with a true IgE-mediated reaction¹:

	Symptoms occur within 1 hour of exposure
	Urticarial, pruritic rash
	Angioedema
	Wheezing
	Shortness of breath
	Anaphylaxis (2 or more organ systems are affected)3,4

If the patient’s rash is nonpuritic and nonurticarial, then the patient did not have an IgE-mediated reaction, and a repeated treatment with the same antibiotic is unlikely to lead to anaphylaxis.⁵ Rashes after administration of a drug are not necessarily IgE-mediated reactions and may be other types such as exanthemous drug reactions.

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“Cross reaction rate between cephalosporins and penicillin is about 0.4%, not 10% as previously believed”

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