Differentiating Viral and Bacterial Pharyngitis
Clinical features alone cannot reliably differentiate viral from bacterial pharyngitis—microbiological confirmation with rapid antigen detection test (RADT) or throat culture is required for suspected bacterial infection. 1, 2
Clinical Assessment Framework
Features Suggesting Viral Etiology
The presence of these features strongly indicates viral pharyngitis and testing for Group A Streptococcus (GAS) is not recommended 2:
- Conjunctivitis 1, 2
- Coryza (runny nose) 1, 2
- Cough 1, 2
- Diarrhea 1, 2
- Hoarseness 2
- Discrete oral ulcers or ulcerative stomatitis 2
Features Suggesting Bacterial (GAS) Etiology
When these features are present, proceed to microbiological testing 1, 2:
- Sudden onset of sore throat 1, 2
- Fever (particularly >38.5°C) 1, 2
- Headache 1
- Nausea, vomiting, or abdominal pain 1
- Tonsillopharyngeal erythema with or without exudates 1, 2
- Tender, enlarged anterior cervical lymph nodes 1, 2
- Palatal petechiae 2
- Absence of cough 1, 2
- Age 5-15 years 1, 2
- Winter or early spring presentation 1, 2
Critical Limitation of Clinical Diagnosis
Even when all clinical features suggest streptococcal pharyngitis, only 35-50% of cases are confirmed to be GAS-positive. 2 The signs and symptoms of bacterial and viral pharyngitis overlap so broadly that accurate diagnosis on clinical grounds alone is impossible, even by experienced physicians. 1, 3
Laboratory Findings (Adjunctive Only)
While laboratory values can provide supportive information, they have poor sensitivity and specificity and should never be used alone to guide antibiotic decisions 2:
Bacterial Pharyngitis Pattern
- Total leukocyte count (TLC) typically >12,000/mm³ 2
- Neutrophilia with left shift (increased band forms) 2
Viral Pharyngitis Pattern
- TLC typically <10,000/mm³ 2
- Relative lymphocytosis 2
- Atypical lymphocytes (especially with Epstein-Barr virus) 2, 4
Recommended Diagnostic Algorithm
Step 1: Initial Clinical Assessment 2, 3
- If obvious viral features (conjunctivitis, coryza, cough, diarrhea) are present → No testing needed, supportive care only
Step 2: Microbiological Testing 1, 2
- If viral features are absent or bacterial features are present → Proceed to RADT or throat culture
- Throat culture remains the gold standard 2
- A positive RADT is diagnostic for GAS pharyngitis 2
- In children and adolescents, negative RADT should be confirmed with throat culture 2
Step 3: Treatment Decision 1
- Only GAS pharyngitis requires antibiotic treatment 1, 3
- Groups C and G streptococci do not have proven benefit from antibiotics 1, 3
Common Pitfalls to Avoid
Relying on clinical impression alone without microbiological confirmation leads to massive antibiotic overuse. 1, 2 Less than half of patients with acute pharyngitis have bacterial infection, yet antibiotics are frequently prescribed based solely on appearance. 1
Do not use TLC/DLC alone to determine antibiotic therapy—this leads to inappropriate treatment decisions. 2
Consider the possibility of GAS carrier state with concurrent viral pharyngitis 2—a positive test may not indicate active GAS infection requiring treatment.
Remember that viruses account for the majority of pharyngitis cases 1, 3, 4, including adenovirus, influenza, parainfluenza, rhinovirus, respiratory syncytial virus, coxsackievirus, herpes simplex virus, and Epstein-Barr virus.
Epidemiological Context
GAS is the most common bacterial cause requiring antibiotic treatment, accounting for 15-30% of pharyngitis cases in children aged 5-15 years. 2, 3 In adults, GAS accounts for less than 15% of cases. 5 GAS pharyngitis is uncommon in children younger than 3 years. 2