Highly Sensitive Molecular Assay for Group A Streptococci Over-Identifies Carriers and May Impact Outpatient Antimicrobial Stewardship.

BACKGROUND: Timely, accurate diagnosis of streptococcal (GAS) pharyngitis prevents ARF and limits antibiotic overuse. The Illumigene® Group A Streptococcus assay (Meridian Bioscience) is a molecular test for GAS pharyngitis with high sensitivity and specificity. We sought to determine whether the Illumigene test is more likely than throat culture to be positive in patients without pharyngeal symptoms and explore the limits of detection of the test.

METHODS: Patients aged 3-17 years were eligible if they had no history of pharyngitis or use of antibiotics within the previous 2 weeks; there were no URI symptoms, sore throat, or fever; and no signs of infection. Culture and Illumigene were performed on duplicate throat swabs. Excess lysate from a subset of Illumigene tests was evaluated by RT-PCR. IRB-approval was obtained.

RESULTS: We enrolled 385 patients from 2/2016 through 10/2017; mean age was 10yrs; 51% were male. Most visits were for health supervision (69%). Significantly more Illumigene tests (78/385, 20.3%) than throat cultures (48/385, 12.5%) were positive (χ, p=.0035). Illumigene was "indeterminate" for 3 patients, leaving 382 pairs of swabs for analysis. Results were discordant for 32/382 pairs (8.4%); 31/32 (97%) were Illumigene-positive/culture-negative (McNemar's test, p<.000001). RT-PCR was negative in 4/13 (31%) tested Illumigene-positive lysates; the paired culture had been negative in all four. The limit of detection for the Illumigene test was 55 CFU/ml.

CONCLUSIONS: The Illumigene test is significantly more likely than throat culture to yield positive results in patients without GAS pharyngitis. Failure to appropriately select patients for testing may negatively impact antimicrobial stewardship efforts without benefit to patients.