NAPLEX Question of the Week: All Things Allergy

This week's question will challenge your knowledge of drug allergies!
NAPLEX Question of the Week: All Things Allergy

PJ is a 57-year-old male with a history of diabetes. His only medication is metformin. Allergies include penicillin (anaphylaxis) and Bactrim (rash). He has avoided the healthcare system and is currently being worked up for a number of potential new diagnoses. 

Which of the following medications if used could potentially cause an allergic reaction due to his history? Select all that apply.

A. Darunavir

B. Azactam

C. Furosemide

D. Lopressor

E. Unasyn

Rationale: An allergy profile is a key component of a patient’s medical history that should be reviewed before selecting appropriate medications for a patient. Allergies to antibiotics are a common self-reported component of the profile and should be evaluated with each episode of care to ensure proper documentation. It is important when assessing allergies to think not only about the obvious contraindicated medications (i.e. Penicillin G in a patient with a penicillin allergy) but also medications that could have cross-reactivity in the patient. A thorough history will often reveal that patients that are reporting penicillin allergy are actually reporting side effects and can be safely de-labeled with patient's consent. 

Answer A is correct. Darunavir is a protease inhibitor used in the management of HIV in combination with other antivirals. It contains a sulfonamide moiety and therefore has the potential for an allergic reaction. In patients that have Bactrim as an allergy listed in their profile, it may not always mean that they could not try another drug with a sulfonamide moiety. For example, patients that experience a mild rash and some itching could be challenged. Other drugs in this class that also have a sulfonamide moiety include fosemprenavir and tipranavir.

Answer B is incorrect. Azactam is a monobactam antibiotic. It possesses a monocyclic beta-lactam structure that does not typically cross-react with penicillins. In rare cases it may cross react with certain cephalosporins (e.g. ceftazidime or cefiderocol). 

Answer C is correct. Furosemide is a loop diuretic. All loop diuretics, including Bumex (bumetanide) and Demadex (torsemide), also have a sulfonamide moiety and have the potential for a reaction. Ethacrynic acid, a previously available loop diuretic that did not contain a sulfa moiety, has been discontinued. In clinical practice, patients with known sulfa allergies to sulfonamide antibiotics can often tolerate well drugs like furosemide if the allergy is mild. If the patient had a severe reaction to Septra or Bactrim, then extreme caution would be warranted and may undergo more rigorous testing by an allergist to determine feasibility of administration. 

Answer D is incorrect. Metoprolol tartrate (Lopressor) is not related structurally to either penicillin or sulfonamides, so it would be appropriate to use for this patient.

Answer E is correct. Unasyn contains ampicillin and sulbactam. Because the patient has reported anaphylaxis to penicillin, ampicillin would definitely have the potential to cause a reaction. Of note, a new product containing sulbactam and durlobactam, brand name of Xacduro, while not containing a penicillin is currently contraindicated in patients with known hypersensitivity to beta-lactams.  Sulbactam, which is used primarily as a beta-lactamase inhibitor in Unasyn, is used in Xacduro as a beta-lactam antibiotic for carbapenem-resistant Acinetobacter baumanii. The durlobactam is used as the beta-lactamase inhibitor in this product. 

Correct Answers: A, C, E

Brand/Generic Names Used: Darunavir (Prezista) or in combination with other antivirals (e.g. Symtuza); Aztreonam (Azactam); Furosemide (Lasix); Metoprolol (Lopressor); Ampicillin/sulbactam (Unasyn); Sulfamethoxazole-trimethoprim (Bactrim or Septra); Metformin (Glucophage)


Darunavir Package Insert –

Aztreonam Package Insert –

Furosemide Package Insert –

Metoprolol Package Insert –,018704s021lbl.pdf

Unasyn Package Insert- 

NAPLEX Competency Areas Covered:

1.4 – From medical records: treatment adherence, or medication-taking behavior; chief complaint, medication history, medical history, family history, social history, lifestyle habits, socioeconomic background

2.1 – Pharmacology, mechanism of action, or therapeutic class

2.2 – Commercial availability; prescription or non-prescription status; brand, generic, or biosimilar names; physical descriptions; or how supplied

3.6 – Drug contraindications, allergies, or precautions


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