One of the most common tests for streptococcal pharyngitis is the rapid antigen detection test (RADT), but is the rapid test for Group A streptococcal (GAS) pharyngitis dependable?

As noted in a new Clinical Infectious Diseases study examining this issue, clinical guidelines state that negative RADT results in adults do not call for confirmation through a backup method, yet lab-based guidelines require confirmation for all age groups. The study authors found that a throat culture may, in fact, be useful after a negative RADT finding.

To determine whether follow-up laboratory cultures after negative RADTs in both adolescents and adults are truly useful, Ferric C. Fang, MD, of the University of Washington School of Medicine in Seattle, and colleagues reviewed the records of 726 patients aged 13 years and older who had had negative RADTs and positive GAS throat cultures. The research team evaluated treatment, complication rates, modified Centor score, and bacterial burden of each patient.

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Fifty-five percent of patients had modified Centor scores of 2 or higher. Of these, 77% had a moderate or heavy bacterial burden. In addition, RADTs had failed to pick up some severe complications: 29 individuals (4%) had peritonsillar abscesses and 2 (0.28%) had a diagnosis of acute rheumatic fever. 

Antibiotic therapy was prescribed for 68.7% of patients; in 43.5% of cases, that treatment was directly prompted by culture results.

The Washington team concluded that reflexive GAS culture is clinically appropriate in the face of negative RADTs—tests that fall short in identifying a large number of people with clinically significant pharyngitis: “Appropriate use of rapid and culture-based diagnostic tests can reduce inappropriate use of antibiotics for sore throats, while avoiding undertreatment of patients who can benefit from antibiotics,” wrote the authors.