What are the diagnostic criteria for pharyngitis?

Diagnostic Criteria for Pharyngitis

The diagnosis of group A streptococcal (GAS) pharyngitis requires laboratory confirmation with either throat culture or rapid antigen detection test (RADT), as clinical features alone cannot reliably distinguish bacterial from viral causes. 1

Clinical Features Suggesting GAS Pharyngitis

The following clinical and epidemiological features increase the probability of GAS infection and should guide testing decisions:

Suggestive Features (Present in GAS)

• Fever (temperature >100.4°F or 38°C) 2, 3
• Tonsillopharyngeal erythema with or without exudates 4
• Tender anterior cervical lymphadenopathy 2, 5
• Absence of cough 2, 5
• Palatal petechiae and beefy red swollen uvula 4
• Scarlatiniform rash 4
• Age 5-15 years (highest risk group) 2, 6
• Close contact with documented GAS case or high community prevalence 1

Features Arguing AGAINST GAS (Suggest Viral Etiology)

• Cough 1, 4, 7
• Rhinorrhea/coryza 1, 4
• Conjunctivitis 1, 4
• Hoarseness 1, 7
• Oral ulcers 4, 7
• Viral exanthem 1
• Diarrhea 3

Clinical Scoring Systems for Risk Stratification

Use the Modified Centor criteria to determine which patients require laboratory testing. 4, 6, 5

Modified Centor Score Components (1 point each):

• Fever (by history or measured) 5
• Tonsillar exudates 5
• Tender anterior cervical adenopathy 5
• Absence of cough 5
• Age adjustment: Age 3-14 years (+1 point); Age 15-44 years (0 points); Age ≥45 years (-1 point) 2

Testing Strategy Based on Score:

• Score 0-1: No testing or treatment needed (low probability of GAS) 6, 5
• Score 2-3: Perform RADT or throat culture; treat only if positive 4, 5
• Score ≥4: May warrant empiric treatment while awaiting test results, or perform RADT/culture 4

Clinical scoring systems have comparable or slightly higher diagnostic accuracy than clinician judgment alone, with the principal utility being identification of low-risk patients to reduce unnecessary testing. 6

Laboratory Confirmation

Throat Culture (Gold Standard)

• Throat culture on sheep blood agar plate remains the diagnostic standard with 90-95% sensitivity 1
• Proper technique is critical: swab both tonsillar surfaces (or fossae) and posterior pharyngeal wall; avoid touching other oral areas 1
• Incubate at 35-37°C for 18-24 hours, then re-examine at 48 hours if initially negative 1
• False negatives occur if antibiotics were recently administered 1

Rapid Antigen Detection Test (RADT)

• Specificity ≥95% (false positives rare) 8
• Sensitivity 80-90% (misses 10-20% of true infections) 8, 4
• Accuracy depends on proper swabbing technique 8

Age-Specific Testing Recommendations

Children and Adolescents:

• Negative RADT must be confirmed with throat culture due to higher prevalence of GAS (20-30%) and risk of rheumatic fever 1, 8, 4
• Treatment can be initiated when culture returns positive; treatment within 9 days of symptom onset still prevents acute rheumatic fever 8

Adults:

• Negative RADT alone is sufficient to rule out GAS; backup culture not necessary 1, 8, 4
• Adults have only 5-10% prevalence of GAS and extremely low rheumatic fever risk 8

When Testing is NOT Indicated

Do not perform testing when clinical features strongly suggest viral etiology (presence of cough, rhinorrhea, conjunctivitis, hoarseness, or oral ulcers). 4, 7

Children under 3 years should not be tested or treated for GAS pharyngitis, as it is rarely involved in this age group. 8

Common Diagnostic Pitfalls to Avoid

• Do not rely on clinical impression alone to diagnose or exclude GAS, as overlap between viral and bacterial presentations is too broad 4
• Do not assume exudates always indicate bacterial infection—exudates can occur with viral pharyngitis 4
• Do not treat based on symptoms alone without laboratory confirmation, as this leads to antibiotic overuse 8
• Do not test or treat asymptomatic household contacts—up to one-third are asymptomatic carriers, and prophylaxis does not reduce subsequent infection rates 8

Management After Negative Testing

Withhold or discontinue antibiotics for patients with negative throat culture or RADT (in adults). 1, 8

Provide symptomatic treatment only: analgesics (acetaminophen or NSAIDs), throat lozenges, and reassurance that symptoms typically resolve in less than 1 week. 8, 7