Questionable penicillin allergy diagnosis increases risk of MRSA and C. difficile 

A retrospective study of outpatient records for millions of  patients in the UK  reveals that  the overuse of broad-spectrum alternative antibiotics among patients who are probably not allergic to penicillin has increased risks of spreading in the general population MRSA (methicillin-resistant Staphylococcus aureus) and Clostridium difficile (C. difficile).

The findings were published online on June 27, 2018 in The BMJ/The British Medical Journal.

“We know that more than 95 percent of patients with an indication of a penicillin allergy in their medical record are found not to be truly allergic, if tested by an allergist,” said author Kimberly Blumenthal, MD, of the Massachusetts General Hospital Division of Rheumatology, Allergy and Immunology in Boston, and assistant professor of medicine at Harvard Medical School.. “Our study identifies appropriate penicillin allergy evaluation – which is still done in less than 1 percent of patients with recorded penicillin allergy – as an essential contributor to the globally important outcomes of antibiotic stewardship, reducing antibiotic resistance and health-care-associated infections.

The investigators analyzed data from The Health Improvement Network (THIN), a database of outpatient primary care medical records from 11.1 million patients in the U.K.

The data covered the period 1995 and 2015.

They noted subjects with a newly documented penicillin allergy. They created a comparator group comprised of up to five subjects for each one in the penicillin allergy group. The comparators were matched for age, sex and time of study entry, and they had received a penicillin prescription during the study period.

Next, the investigators evaluated medical records of all subjects for any documented diagnoses of MRSA or C. difficile.

They found that, during the period of the study, subjects diagnosed with a penicillin allergy had a 69 percent greater risk of contracting MRSA than did subjects in the comparison group and a 26 percent greater risk of a C. difficile diagnosis. Both findings held after the investigators evaluated the data for other known risk factors for either infection.

The investigators reported that over half of the increased MRSA risk and 35 percent of the increased C. difficile risk was attributable to use of the antibiotic alternatives beta-lactams (the drug class that includes penicillin) which were given to subjects  patients believed to be allergic to penicillin. Prior research has shown that use of such broad-spectrum antibiotic alternatives increase the incidence of antibiotic resistance and increases raise the risk of more virulent infections like C. difficile.

“Both MRSA and C. difficile infections are increasing public health burdens, are challenging to treat and cause the deaths of thousands of patients every year,” said Blumenthal.  “We now can see a causal path from patients being labeled as having a penicillin allergy, to their being prescribed beta-lactam alternative antibiotics, to greater incidence of these dangerous and costly infections. MRSA and C. difficile each cost our health care system more than $1 billion in direct costs annually, so the modest cost of penicillin allergy evaluation – around $220 per patient – seems greatly worthwhile.”