Infectious Disease
Group A Streptococcus
Group A Streptococcus (Streptococcus pyogenes) is a bacterium tested at the point of care to support the assessment of acute sore throat and pharyngitis. Near-patient assays detect either streptococcal carbohydrate antigen or bacterial DNA directly from a throat swab, giving a same-visit qualitative result.
Why it is measured
Rapid identification of Group A Streptococcus helps clinicians target antibiotic therapy and supports antimicrobial stewardship by distinguishing likely bacterial pharyngitis from viral causes. It can also reduce the wait for laboratory throat culture, which typically takes a day or more.
| Typical range | Qualitative result, not a numeric concentration: the expected result in the absence of infection or carriage is Negative (not detected). A Positive (detected) result indicates streptococcal antigen or DNA is present. There are no measurement units. Reporting conventions, analytical sensitivity and the recommendation to culture-confirm negative antigen results vary by method and local guideline; molecular assays are generally more sensitive than lateral-flow antigen tests. |
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| Sample | Throat swab from the posterior pharynx and both tonsils (tonsillar pillars), avoiding the tongue, cheeks and teeth. Some molecular platforms accept a single swab; confirmatory culture may use a separate swab. |
| Turnaround | Rapid antigen and digital immunoassays typically report in about 5 to 10 minutes; near-patient molecular (PCR or isothermal) assays typically report in about 2 to 25 minutes depending on the platform, with some isothermal systems flagging positives within a few minutes. |
Point of care devices that report it
- Abbott ID NOW Strep A 2 (isothermal molecular)
- Roche cobas Liat Strep A (real-time PCR)
- Cepheid GeneXpert Xpert Xpress Strep A (real-time PCR)
- QuidelOrtho Sofia Strep A+ FIA (fluorescent immunoassay)
- BD Veritor Plus System Group A Strep (digital immunoassay)
- QuidelOrtho Solana Group A Streptococcus (helicase-dependent amplification, near-patient molecular)
- Abbott BinaxNOW Strep A (lateral-flow antigen)
- Sekisui OSOM Strep A (lateral-flow antigen)
Questions, answered
Why might a negative rapid antigen test still be followed by a throat culture?
Lateral-flow antigen tests are highly specific but can be less sensitive than culture or molecular methods, so a negative antigen result in someone with suggestive symptoms may be confirmed by throat culture or a molecular assay, in line with local guidance. This is a general point about test performance and not advice about any individual patient.
What is the difference between antigen and molecular Group A Strep point-of-care tests?
Antigen tests (lateral-flow or fluorescent and digital immunoassays) detect the streptococcal carbohydrate antigen and are typically very fast and low cost. Molecular tests (PCR or isothermal amplification) detect bacterial DNA and are generally more sensitive, often removing the need for routine culture confirmation of negatives, but usually cost more per test.
Does a positive Group A Strep test always mean active infection needing antibiotics?
A positive result confirms that Group A streptococcal antigen or DNA is present, but it cannot by itself distinguish active infection from asymptomatic carriage. Interpretation should always combine the result with the clinical picture and relevant guidelines, which is a matter for the treating clinician rather than the test alone.