A 28-year-old woman presents with acute pharyngitis, fever, and cervical lymphadenopathy. Throat swab culture is pending. Which investigation is most appropriate to confirm Streptococcus pyogenes infection and guide immediate antibiotic therapy?

## Investigation of Choice for Acute S. pyogenes Pharyngitis **Key Point:** Rapid antigen detection test (RADT) is the gold standard for rapid diagnosis of acute Group A Streptococcus (GAS) pharyngitis in clinical practice. It provides results within 5–15 minutes, enabling immediate treatment initiation. ### Why RADT is Preferred **High-Yield:** RADT detects Group A carbohydrate antigen directly from throat swab using immunochromatographic or enzyme immunoassay methods. Sensitivity ranges from 90–95%, and specificity is >95%. **Clinical Pearl:** RADT allows same-visit diagnosis and treatment, reducing disease transmission and preventing suppurative complications (peritonsillar abscess, retropharyngeal abscess) and post-infectious sequelae (acute rheumatic fever, post-streptococcal glomerulonephritis). ### Comparison with Other Investigations | Investigation | Timing | Sensitivity | Specificity | Clinical Use | |---|---|---|---|---| | **RADT** | 5–15 min | 90–95% | >95% | **Acute diagnosis, guides immediate therapy** | | Throat culture (blood agar + bacitracin) | 24–48 hrs | 95–98% | 99% | Gold standard; used when RADT negative but clinical suspicion high | | Blood culture | 24–48 hrs | Low (~5%) | High | Reserved for invasive disease (cellulitis, necrotizing fasciitis) | | ASO titre | Days–weeks | — | — | Retrospective diagnosis of post-streptococcal sequelae, not acute infection | **Tip:** If RADT is negative but clinical suspicion remains high (e.g., scarlet fever rash, severe symptoms), throat culture should be obtained as a confirmatory test. ### Diagnostic Algorithm ```mermaid flowchart TD A[Acute pharyngitis suspected]:::outcome --> B[Perform RADT]:::action B --> C{RADT positive?}:::decision C -->|Yes| D[Start antibiotics immediately]:::action C -->|No| E{High clinical suspicion?}:::decision E -->|Yes| F[Throat culture on blood agar + bacitracin]:::action E -->|No| G[Supportive care, no antibiotics]:::action F --> H{Culture positive?}:::decision H -->|Yes| D H -->|No| G ``` [cite:Harrison 21e Ch 143]