Identifying the true cause of a life-threatening allergic reaction during surgery remains one of medicine's trickiest diagnostic challenges. When anaphylaxis strikes on the operating table, patients often receive multiple medications within minutes, making it nearly impossible to pinpoint which drug triggered the emergency. Even with detailed timelines and comprehensive testing, allergists can confirm a culprit in only about one-third to one-half of cases referred for evaluation after surgery.

Why Do Drug Allergy Labels Complicate Surgical Planning?

Most patients heading into the operating room carry at least one documented drug allergy label in their medical chart. These warnings can stem from a forgotten childhood reaction to penicillin, an adverse effect from ibuprofen, or a past experience with an opioid. While some represent true allergies, others do not, yet all warrant careful evaluation before surgery.

Preoperative teams pay closest attention to allergy labels affecting anesthesia choices. A reported history of reactions to medications such as lidocaine, propofol, fentanyl, or midazolam can complicate anesthetic planning and influence which drugs clinicians select. Beyond the operation itself, teams must also plan for postoperative recovery, ensuring safe pain management options if a patient reports allergies to NSAIDs or opioids.

Antibiotic allergy labels are equally critical because most surgical patients receive preventive antibiotics to reduce infection risk at the surgical site. Cefazolin, commonly known by the brand name Ancef, is the preferred first-line antibiotic for many common procedures. However, patients who report childhood reactions to related antibiotics such as cephalexin or cefadroxil often raise questions about whether cefazolin can be used safely.

"[Collecting] the allergy history for all patients with drug allergy labels before surgery is important. Things that have significant reactions you might want to avoid. Things that are lower risk, [or] moderate risk, we could potentially test using a combination of skin testing and drug challenge tests," explained Kimberly Blumenthal, MD.

Kimberly Blumenthal, MD, Allergy Specialist

What Does Research Show About Penicillin and Cefazolin Cross-Reactivity?

Growing evidence shows there is no meaningful cross-reactivity between penicillin and cefazolin specifically. As a result, nearly all patients with a penicillin allergy label can safely receive cefazolin in the operating room after appropriate clinical evaluation. In fact, research has shown that avoiding first-line cefazolin because of an unverified penicillin allergy label can actually increase a patient's risk of developing a surgical site infection.

How to Prepare for Surgery When You Have a Drug Allergy History

• Collect a Detailed History: Before surgery, work with your preoperative team to document exactly what happened during past reactions, including symptoms, timing, and which medications were involved at that moment.
• Request Allergy Testing: Ask whether skin testing or drug challenge tests can help clarify whether you truly have an allergy or experienced a different type of adverse reaction.
• Discuss Antibiotic Options: If you have a reported penicillin allergy, specifically ask your surgical team whether cefazolin can be used safely after evaluation, since it may be the best infection-prevention option for your procedure.

What Happens When Anaphylaxis Occurs During Surgery?

When perioperative anaphylaxis does occur, figuring out exactly what caused it becomes a complex medical investigation. Unlike a controlled allergy challenge in a clinic, where one medication is introduced at a time, patients undergoing surgery often receive multiple drugs within just a few minutes. To untangle the sequence of events, allergists reconstruct a minute-by-minute timeline from the anesthesiologist's record to determine which medication was administered immediately before the reaction began.

Even with a detailed timeline, identifying the responsible drug remains difficult. In practice, comprehensive allergy testing identifies a likely causative agent in only about one-third to one-half of patients referred for postoperative evaluation by an allergist.

When a cause is identified in the United States, antibiotics, with cefazolin leading the list, are the most common trigger of perioperative anaphylaxis. Neuromuscular blocking agents, the medications used to produce temporary muscle paralysis during surgery, rank second. These drugs present a unique diagnostic challenge because many reactions appear to result from direct mast cell activation rather than the classic IgE-mediated allergic pathway, making them more difficult to confirm through standard allergy testing.

Ultimately, careful allergy evaluation before surgery and thorough investigation afterward go hand in hand. Identifying which drug truly caused a reaction not only helps explain what happened during a patient's procedure but also allows clinicians to plan future surgeries more safely while preventing patients from carrying inaccurate drug allergy labels that could unnecessarily limit their treatment options for years to come.