Penicillin Allergy

20F p/w pharyngitis w/fever. There is no cough. Exam: Cervical adenopathy; tonsillar exudate. Rapid Strep antigen test pos. You start to prescribe her Amoxicillin but there is an allergy alert.

She reports an allergic reaction to penicillin around age 8 or 9. She had a rash but no other symptoms. It resolved following discontinuation of med. She did not receive any treatment. Which of the following would you do next?

Today we are going to talk about everyone’s favorite – #penicillin #allergy!

Excited GIF

Penicillin is the most commonly reported drug allergy (followed by sulfonamides) documented in patient charts. However, most of these are documented based on patient account and not observed reactions. PMID: 26970431

PCN allergy is assc w/worse outcomes. The avoidance of beta-lactams when they are the reatment of choice, has been associated with incr. risk of drug resistant infections, C difficile colitis and increased hospital LOS.
PMID: 29950489; 24188976; 27402820

How does penicillin cause allergy? This great review from @NEJM (PMID: 31826341) explains. The beta-lactam ring has the ability to bind lysine residues in the plasma and can form penicilloyl polylysine, which stimulates antibody production.

Following sensitization, rechallenge of penicillin can lead to IgE mediated antibody response, which leads to mast cell degranulation and the classic presentation of anaphylaxis. Source: NEJM

Penicillin, and beta-lactams in general, are also associated with other types of allergic reactions, including T Cell-mediated (Type IV), IgG-mediated (Type II), and immune complex-mediated (Type III) reactions. However, anaphylaxis is the most feared adverse effect.

Penicillin allergy can be deadly, but a significant proportion of people do not have real allergy when tested or re-challenged. This equates to worsened patient outcomes for no reason. PMID: 30644987 Image (skin testing) from JAMA – PMID: 30644984

How can you take PCN allergy off of pts’s charts? One way is to perform skin testing, which has been shown to be safe and effective (PMID: 24565482). Alternatively, a recently developed decision support tool called PEN-FAST can help to delabel patients (PMID: 32176248).

PEN-FAST was derived from data on > 600 pts and was validated in > 900 pts. A score < 3 equates to a NPV of 96.3% and is suggested a cutoff for possible oral challenge w/skin testing. In this scenario our patient had a score of 0, w/NPV of 99.4%.

https://qxmd.com/calculate/calculator_752/pen-fast–penicillin-allergy-risk-tool

In our patient, the chance of any hypersensitivity response would have been approx 0.6%.

Originally tweeted by Infectious Diseases Fellows Network (@ID_fellows) on 20 November, 2020.

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