Rapid tests for group A strep pharyngitis generally accurate

By Megan Brooks

NEW YORK (Reuters Health) - Rapid antigen diagnostic tests in general have high accuracy for diagnosis of group A streptococcal (GAS) pharyngitis in children with sore throat, and backup throat culture isn't needed in most settings with a low incidence of rheumatic fever, researchers from Australia report.

Sore throat is a common complaint, especially in children, and making a diagnosing of GAS pharyngitis can be challenging, Dr. Andrew Steer of the Royal Children's Hospital in Melbourne and colleagues note in a paper online September 8 in Pediatrics.

The signs and symptoms of GAS pharyngitis are often indistinguishable from viral and other causes of sore throat, they explain, and no symptom or sign in isolation has been shown to have a sufficiently high likelihood ratio to allow accurate diagnosis.

Effective management of GAS pharyngitis is "hindered by impracticality of the gold standard diagnostic test: throat culture," they point out. "Rapid antigen diagnostic tests (RADTs) are a promising alternative, although concerns about their sensitivity and specificity, and variation between test methodologies, have limited their clinical use."

The researchers assessed the diagnostic accuracy of RADTs for GAS pharyngitis using throat culture on blood agar as a reference standard in a meta-analysis with data from 48 studies (60 pairs of sensitivity and specificity).

"Our findings suggest that overall these tests perform reasonably well, but we observed considerable variability between individual tests," Dr. Steer commented in email to Reuters Health.

Overall, the RADTs had a sensitivity of 0.86 (95% CI, 0.83 to 0.88) and specificity of 0.96 (95% CI 0.94 to 0.97), the authors report.

"The newer molecular-based tests appear to have the highest sensitivity but their clinical use is currently limited by their slower turn-around-time (TAT) compared to the immunoassay-based tests," Dr. Steer told Reuters Health.

Current U.S. guidelines recommend RADTs for the diagnosis of GAS pharyngitis in children at least three years old and adults with acute sore throat who do not have signs suggestive of a viral etiology. In children, the guidelines recommend backup culture for negative RADTs.

"We believe that in regions where rheumatic fever is uncommon that a RADT is an appropriate test and that if it is negative then a backup culture is generally unnecessary. We make this statement on the basis that the overall sensitivity of RADT is around 85%. An important caveat to this statement is that clinicians need to be aware of the performance of the test that they use, particularly its sensitivity," Dr. Steer said.

"In regions where rheumatic fever is common, we believe that RADTs are not yet sufficiently sensitive and so a backup culture is necessary. We hope that newer molecular-based tests, which appear to have a high sensitivity, will be developed with a faster TAT and that these tests will be able to be used in rheumatic fever endemic countries," Dr. Steer added.

The authors note in their paper that routine use of confirmatory cultures to back up RADTs has been shown to cost more than $8 million per additional case of rheumatic heart disease prevented, and "should be questioned as a cost-effective approach to management."

The study had no external funding. Dr. Steer and a co-author received funding for a clinical study conducted in 2012 of a Quidel Corporation RADT.

SOURCE: http://bit.ly/1rwrdow

Pediatrics 2014.

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