Evaluating Streptococcal Pharyngitis Likelihood
Use clinical and epidemiological features to determine who needs testing, but laboratory confirmation is essential for diagnosis because clinical judgment alone is insufficiently accurate—even experienced physicians cannot reliably distinguish streptococcal from viral pharyngitis based on symptoms alone. 1
Clinical Features That Increase Strep Likelihood
Features suggesting streptococcal infection:
- Sudden onset sore throat with pain on swallowing 1
- Fever (absence of fever suggests viral etiology) 1
- Tonsillopharyngeal erythema with or without exudates 1
- Tender enlarged anterior cervical lymph nodes 1
- Palatal petechiae 1
- Scarlatiniform rash 1
- Headache, nausea, vomiting, or abdominal pain (especially in children) 1
Features strongly suggesting viral (not streptococcal) etiology:
- Cough, hoarseness, or coryza 1
- Conjunctivitis 1
- Anterior stomatitis or discrete ulcerative lesions 1
- Viral exanthem 1
- Diarrhea 1
Who Should Be Tested
Do NOT test patients when:
- Clinical and epidemiological features strongly suggest viral etiology (presence of cough, rhinorrhea, hoarseness, oral ulcers) 1
- Children under 3 years old (streptococcal pharyngitis and acute rheumatic fever are uncommon in this age group), unless high-risk factors exist such as an older sibling with confirmed GAS infection 1
DO test patients when:
- Clinical features do not clearly exclude streptococcal infection 1
- History of close contact with documented streptococcal pharyngitis 1
- High community prevalence of group A streptococcal infections 1
- Age 5-15 years during winter/early spring months 1
Clinical Scoring Systems
Clinical prediction rules can help stratify risk but have significant limitations:
- Scoring systems predict positive throat cultures with only ≤80% accuracy at best 1
- Therefore, bacteriologic confirmation is required unless streptococcal pharyngitis can be confidently excluded on clinical and epidemiologic grounds 1
- The modified Centor score is the most commonly used system, incorporating fever, tonsillar exudate, tender anterior cervical lymphadenopathy, and absence of cough 2, 3
- Physicians consistently overestimate the probability of streptococcal pharyngitis without laboratory confirmation 1
Laboratory Confirmation Strategy
Throat culture remains the gold standard:
- Single throat swab on sheep blood agar has 90-95% sensitivity for detecting group A streptococci 1
- Swab must be obtained from both tonsils (or tonsillar fossae) AND posterior pharyngeal wall—other oropharyngeal areas are unacceptable 1
- False negatives occur if antibiotics were given shortly before or during specimen collection 1
Rapid antigen detection tests (RADTs):
- Sensitivity <90% in most pediatric populations 1
- In children and adolescents: negative RADT requires backup throat culture 1
- In adults: negative RADT without backup culture is acceptable because GAS causes only 5-15% of adult pharyngitis cases and acute rheumatic fever risk is extremely low 1
Age-Specific Considerations
Pediatric patients (ages 5-15):
- GAS causes 20-30% of acute pharyngitis cases 1
- Always perform backup throat culture after negative RADT 1
Adults:
- GAS causes only 5-15% of acute pharyngitis cases 1
- Risk higher in parents of school-age children or those working closely with children 1
- Negative RADT without backup culture is acceptable due to low prevalence and minimal rheumatic fever risk 1
Critical Pitfall to Avoid
The most important pitfall: Up to 70% of patients with sore throats receive antibiotics, yet only 20-30% actually have GAS pharyngitis 1. Selective testing based on clinical features increases the proportion of true positives and reduces unnecessary antibiotic prescriptions, which is critical given rising antimicrobial resistance 1.