Strep detection tests may miss sore throat-causing bacterial strains

Rapid streptococcal antigen detection tests (RADTs), which are widely used to detect non-group A Lancefield beta-hemolytic streptococci (GABHS) in many developed countries, may be missing clinically important streptococci, say researchers.

Paul Little (University of Southampton, UK) and colleagues found that GABHS strains commonly cause streptococcal sore throats and present with similar symptomatic clinical features as group A streptococci.

"The best features to predict streptococcal sore throat presenting in primary care deserve revisiting since features not commonly used in diagnosis (eg, rapid presentation, severity of inflammation) may be useful clinically," they write in the British Journal of General Practice.

The prospective diagnostic cohort study showed that pathogenic streptococci were found in the throat swabs of 34% (n=204) of a cohort of 597 UK primary care patients with a history of sore throat (< 2 weeks) and an abnormal throat examination. Of these cases, 33% were non-group A streptococci and were mostly group C (n=29) followed by G (n=18), and B (n=17). Cases of group D streptococci and Streptococcus pneumoniae were rare (n=3 and n=1, respectively).

Patients presented with similar features whether the streptococci were group A or non-Group A, the authors note.

The features best predicting the presence of A, C, or G beta-hemolytic streptococci were the patient's assessment of severity (odds ratio [OR]=3.31 for bad sore throat) and doctor's assessment of severity (OR=2.28 for severely inflamed tonsils). Other predictors were the absence of a bad cough (OR=2.73), absence of a coryza (OR=2.73), and moderately bad or very bad muscle aches (OR=2.20).

"The study supports previous observations that the presence of group C and G streptococci is unlikely to be due to commensal carriage," remark Little and team. "Group C and G streptococci are common (approaching 25% of streptococci) and present with symptomatic illness in a similar manner to group A streptococci which raises the issue of whether RADTs should be used to exclude the presence of pathogenic streptococci."

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