## Diagnosis of Streptococcus pyogenes Pharyngitis ### Gold Standard Investigation **Key Point:** Throat culture on blood agar followed by bacitracin susceptibility testing remains the gold standard for confirming *S. pyogenes* infection, especially when RADT is negative but clinical suspicion remains high. ### Methodology 1. **Culture medium**: Blood agar (5% sheep blood) 2. **Incubation**: 35–37°C, 5% CO₂, 24–48 hours 3. **Colony morphology**: Small, translucent, β-hemolytic colonies 4. **Confirmatory test**: Bacitracin susceptibility (0.04 units disk) - *S. pyogenes* is bacitracin-sensitive (zone of inhibition ≥12 mm) - Differentiates from *S. agalactiae* (bacitracin-resistant) ### Why Culture Over RADT? - RADT has 90–95% sensitivity but 5–10% false-negative rate - Culture detects carriers and low-burden infections missed by RADT - Allows antimicrobial susceptibility testing if needed - Provides definitive identification via Lancefield grouping (Group A Streptococcus) **Clinical Pearl:** In a patient with negative RADT but high clinical suspicion (fever, exudate, pharyngeal erythema), culture is warranted to avoid missing treatable infection and potential rheumatic fever sequelae. ### Comparison of Diagnostic Methods | Investigation | Sensitivity | Specificity | Turnaround Time | Use Case | |---|---|---|---|---| | RADT | 90–95% | 95–99% | 5–15 min | Rapid point-of-care screening | | Throat culture + bacitracin | 95–99% | 99% | 24–48 hrs | Gold standard; negative RADT follow-up | | ASO titre | N/A | N/A | 1–2 weeks | Post-streptococcal sequelae (ARF, PSGN) | | Latex agglutination | 85–90% | 95% | 30 min | Rapid but less sensitive than culture | **High-Yield:** Bacitracin sensitivity is the defining biochemical test for *S. pyogenes* identification in clinical microbiology labs.
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