Confirmation Required for Negative Rapid Group A Streptococcal Test

Negative tests results from a rapid analysis of a throat swab for Group A streptococcal (GAS) pharyngitis, performed in a doctor's office should be confirmed through a follow-up laboratory culture.

Clinical guidelines state that negative rapid antigen detection tests (RADTs) for GAS do not require confirmation by a back-up method in adults, whereas laboratory-based guidelines mandate confirmation of a negative RADT in patients of all ages.

Scientists at the University of Washington Medical Center (Seattle, WA, USA) carried out a retrospective analysis of 726 patients, aged 13 and older, with negative RADTs and positive GAS throat cultures performed between January 1, 2000 and Dec 31, 2011, at two academic medical centers in Seattle. Complication rates, treatment, modified Centor score and bacterial burden in patients with negative RADTs and positive GAS throat cultures were assessed.

Modified Centor scores greater than or equal to 2 were observed in 55% of patients with a negative RADT and positive GAS culture. Of these, 77% of patients had a moderate or heavy bacterial burden, equal to or greater than 2+. RADTs failed to detect some patients who presented with serious complications of GAS pharyngitis as 29 (4.0%) had peritonsillar abscesses and two (0.28%) were diagnosed with acute rheumatic fever.

The investigators found culture results to be useful for initiating antibiotic therapy or confirming a clinical diagnosis. Antibiotic treatment was prescribed in 68.7% of patients, with culture-directed initiation of therapy documented in 43.5%. The scientists found that more than 1,000 patients whose rapid throat cultures were negative actually turned out to have Group A Streptococcus in their laboratory throat cultures. Streptococcus pyogenes, was the bacteria associated with Group A streptococcal pharyngitis.

The authors concluded that appropriate use of rapid and culture-based diagnostic tests can reduce inappropriate use of antibiotics for sore throats, while avoiding under-treatment of patients who can benefit from antibiotics. Ferric C. Fang, MD, professor of microbiology and laboratory medicine and senior author of the study said, “Each year nearly seven million Americans seek medical attention for a sore throat, making it one of the most common reasons to see a doctor.” The study was published on May 27, 2014, in the journal Clinical Infectious Diseases.

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University of Washington Medical Center