A 22-year-old female presents with a 2-day history of acute pharyngitis with fever, sore throat, and mild dysphagia. On examination, bilateral tonsils are enlarged with white exudate. There is no uvula deviation or bulging of the soft palate. Which investigation is most appropriate to differentiate bacterial from viral tonsillitis and guide antibiotic therapy?

Explanation

## Differentiating Bacterial from Viral Tonsillitis **Key Point:** Acute bacterial tonsillitis (most commonly caused by Group A Streptococcus [GAS]) requires antibiotic therapy, while viral tonsillitis is self-limiting. Rapid and accurate differentiation is essential to avoid unnecessary antibiotic use and reduce antimicrobial resistance. ### Clinical Features and Diagnosis | Feature | Viral Tonsillitis | Bacterial (GAS) Tonsillitis | |---------|-------------------|-----------------------------| | Onset | Gradual | Acute | | Exudate | Variable | Yellow/white, purulent | | Cervical lymph nodes | Mild | Tender, enlarged | | Fever | Moderate | High (>38.5°C) | | Cough | Present | Absent | | Diagnosis | Clinical/PCR | RADT or throat culture | | Treatment | Supportive | Antibiotics (Penicillin V or Amoxicillin) | **High-Yield:** The **Centor criteria** help predict GAS tonsillitis probability: 1. Fever >38.5°C 2. Absence of cough 3. Tender cervical lymphadenopathy 4. Tonsillar exudate Scores ≥3 suggest GAS infection and warrant RADT/culture. ### Role of RADT in Acute Tonsillitis **Clinical Pearl:** Rapid Antigen Detection Test (RADT) for Group A Streptococcus is the **investigation of choice** for acute bacterial tonsillitis because: - **Sensitivity:** 85–95% (rapid results in 5–10 minutes) - **Specificity:** >95% - **Cost-effective** and widely available - **Guides immediate antibiotic therapy** - **Reduces unnecessary antibiotic use** in viral cases ```mermaid flowchart TD A[Acute pharyngitis/tonsillitis]:::outcome --> B{Centor score ≥3?}:::decision B -->|Yes| C[Perform RADT for GAS]:::action B -->|No| D[Likely viral<br/>Supportive care]:::action C --> E{RADT positive?}:::decision E -->|Yes| F[Start antibiotic<br/>Penicillin V or Amoxicillin]:::action E -->|No| G[If high clinical suspicion:<br/>Throat culture confirmation]:::action F --> H[Prevents complications<br/>Reduces transmission]:::outcome ``` **Mnemonic:** **RADT** = **R**apid **A**ntigen **D**etection **T**est — the rapid, point-of-care test for GAS tonsillitis. ### Why Other Investigations Are Not First-Line **Throat culture** is more sensitive (95–99%) but takes 24–48 hours; RADT is preferred for immediate management. If RADT is negative but clinical suspicion is high, culture can confirm GAS. **Viral PCR panel** identifies viral pathogens (rhinovirus, adenovirus, EBV, CMV) but does not guide acute management and is expensive. Reserved for cases with atypical features or immunocompromised patients. **CBC with differential** may show leukocytosis in bacterial infection, but it is non-specific and does not differentiate GAS from other bacteria or viruses. It is not used for diagnostic decision-making in acute tonsillitis. **CT scan** is reserved for suspected complications (peritonsillar abscess, retropharyngeal abscess) — not for uncomplicated acute tonsillitis. **Tip:** In clinical practice, RADT + throat culture is the gold standard: RADT for rapid diagnosis, culture for confirmation if RADT is negative but clinical suspicion remains high. [cite:Harrison's Principles of Internal Medicine 21e Ch 143]