From the Guidelines
An allergy to Omnicef (cefdinir) does not necessarily create a hard contraindication to Ancef (cefazolin), as cross-reactivity between different generations of cephalosporins is generally low, around 1-10% 1.
Key Considerations
- Patients with a history of mild reactions to Omnicef (such as rash) may often tolerate Ancef safely, but caution is warranted, and clinical judgment is essential.
- If the previous reaction to Omnicef was severe (anaphylaxis, Stevens-Johnson syndrome, or other life-threatening reaction), then Ancef should be avoided as the risk of cross-reactivity becomes more concerning.
- In cases of mild previous reactions, some clinicians may choose to administer Ancef with careful monitoring, possibly starting with a test dose.
Rationale
- The cross-reactivity occurs because cephalosporins share a similar beta-lactam ring structure, but differences in side chains between various cephalosporins affect the likelihood of cross-reactivity 1.
- A recent study found that the reaction rate among patients with a confirmed penicillin allergy was just 0.8% (95% CI: 0.13%-4.1%) among 131 patients who are confirmed to be penicillin-allergic 1.
- Another study found that the cross-reactivity rate to cefazolin among patients with an unverified penicillin allergy is 0.7% (95% CrI: 0.1%-1.7%) 1.
Clinical Implications
- The decision to administer Ancef to a patient with a history of Omnicef allergy should be individualized based on the severity of the prior reaction, the necessity of the antibiotic, and available alternatives.
- Clinicians should exercise caution and consider the potential risks and benefits of administering Ancef to a patient with a history of Omnicef allergy.
- Further evaluation, such as skin testing, may be necessary in some cases to determine the safety of administering Ancef to a patient with a history of Omnicef allergy 1.
From the FDA Drug Label
CEFAZOLIN IS CONTRAINDICATED IN PATIENTS WITH KNOWN ALLERGY TO THE CEPHALOSPORIN GROUP OF ANTIBIOTICS. There is a contraindication to Cefazolin (Ancef) use in patients with a known allergy to the cephalosporin group of antibiotics. Since Cefdinir (Omnicef) is a cephalosporin antibiotic, patients with a Cefdinir allergy may be at risk. Therefore, Cefazolin is contraindicated in patients with a Cefdinir allergy, due to the risk of cross-reactivity within the cephalosporin group 2.
From the Research
Cephalosporin Allergy and Cross-Reactivity
- Cephalosporins are a class of antibiotics that are commonly prescribed due to their wide clinical utility and general tolerability 3.
- The cross-reactivity between cephalosporins and other antibiotics, such as penicillins, is a concern for clinicians when prescribing antibiotics to patients with a history of allergy 4, 5.
- Studies have shown that the cross-reactivity between cephalosporins and penicillins is lower than previously reported, with an overall cross-reactivity rate of approximately 1% when using first-generation cephalosporins or cephalosporins with similar R1 side chains 5.
Cefazolin and Penicillin Allergy
- Cefazolin, a first-generation cephalosporin, is often used in surgery to treat or prevent bacterial infection of the surgical site 6.
- There is no direct evidence to suggest a contraindication to Cefazolin use in patients with a Cefdinir (Omnicef) allergy, as Cefdinir is a third-generation cephalosporin and Cefazolin is a first-generation cephalosporin.
- However, patients with a penicillin allergy may have a higher risk of cross-reactivity with first-generation cephalosporins like Cefazolin, although the risk is still considered low 4, 5.
R1 Side Chain and Cross-Reactivity
- The R1 side chain of cephalosporins is responsible for cross-reactivity with penicillins, and clinicians should be aware of R1 side chain similarities when prescribing alternate β-lactams in allergic individuals 3, 5.
- Later-generation cephalosporins, which have distinct side chains, do not have any skin test cross-reactivity with penicillin/amoxicillin 7.