Health

Widely used antibiotic allergy skin test often wrong, doctors say

Most people who believe they're allergic to penicillin truly aren't, say Canadian doctors who stress both children and adults should be diagnosed properly because the substitutes are more toxic and expensive.

Penicillin allergy often either misdiagnosed or outgrown and should be tested properly

Researchers in Montreal say skin tests used to diagnose allergies to a widely prescribed antibiotic are practically useless 2:24

Most people who believe they're allergic to penicillin truly aren't, say Canadian doctors who stress both children and adults should be diagnosed properly because substitutes for antibiotics like penicillin and amoxicillin are more toxic and expensive.

People may think they're allergic to antibiotics in the same class as penicillin because they developed a rash after taking it and were never evaluated.

But studies suggest at least 80 per cent of adults and at least 90 per cent of children with claims of penicillin allergy can receive the antibiotic, said Dr. Gordon Sussman, an allergist and immunologist in Toronto.

If you're unnecessarily tagged as allergic and you're not truly allergic, you will be avoiding the first line treatment for your condition and instead replace it with more expensive and less effective medications.- Dr. Moshe  Ben-Shoshan

"People should be aware that when they're diagnosed with a penicillin allergy, either you outgrow it or maybe it's a virus that was misdiagnosed," Sussman said in an interview. "At some point it should be tested properly."

It's important to properly diagnose penicillin allergy for several reasons, said Dr. Moshe Ben-Shoshan, an assistant professor in allergy and immunology at Montreal Children's Hospital.

 "I think it's very important because for those who do have allergies, we don't know the severity of subsequent reactions so it's important to establish the true presence of allergy," Ben-Shoshan said. "The other side of the coin is if you're unnecessarily tagged as allergic and you're not truly allergic, you will be avoiding the first line treatment for your condition and instead replace it with more expensive and less effective medications."

A type of ear infection called acute otitis media is a common reason for medical visits among North American children. Even though observation is often recommended, amoxicillin is commonly prescribed, studies in the U.S and Quebec suggest.

In 10 per cent of cases of allergies to amoxicillin among children a rash is reported, Ben-Shoshan said.

In the April issue of JAMA Pediatrics, Ben-Shoshan reported on the poor predictive value of skin tests for screening for amoxicillin allergy.

'Emperor has no clothes' for skin test

The gold standard way to test for an allergy is an oral challenge.

Ben-Shoshan and his team studied 818 children aged one to nearly four who were referred to his hospital between 2012 and 2015 with a rash and suspected history of allergy to penicillin. They were followed for up to three months.

The children were given a "graded oral provocation challenge" — 10 per cent of the full dose and then the rest 20 minutes later.  

Of the 17 children who reacted immediately, the follow-up skin test was positive in only one child. If the skin test was appropriate measure, it should've been positive in all 17, the researchers said.

"The emperor has no clothes here so the skin test is not useful and is a poor diagnostic test for those with immediate reactions to amoxicillin," Ben-Shoshan said. Amoxicillin is in the same class of antibiotics as penicillin.

For children presenting with a rash like those in the study, Ben-Shoshan suggests skipping skin tests and going directly to the challenge in hospital to save time and cost to the health-care system. 

Sussman said skin tests still have a place.

For people who have had hives or a dangerous reaction like anaphylaxis, Sussman still recommends a skin test because it's safer for patients.

A journal editorial published with the study also said it's important to identify children who are not allergic, because those who are mislabelled have more medical visits, more antibiotic prescriptions, longer hospitalizations and more antibiotic-related complications, based on previous studies.

Doctors have also found adults who were tested for antibiotic allergies before surgery such as cardiac procedures and turned out to be not allergic. 

With files from CBC's Amina Zafar

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