What is the best method to diagnose strep throat in patients with symptoms of pharyngitis?

How to Test for Strep Throat

Use a rapid antigen detection test (RADT) as your first-line diagnostic test, and if negative in children/adolescents, follow up with a throat culture—but in adults, a negative RADT alone is sufficient without backup culture. 1

Specimen Collection Technique

Proper swabbing technique is critical to test accuracy:

• Swab both tonsils (or tonsillar fossae) and the posterior pharyngeal wall vigorously 1
• Do not touch other areas of the oropharynx or mouth before or after sampling the appropriate sites 1
• Avoid testing patients who have recently received antibiotics, as this causes false-negative results 1, 2

Testing Algorithm by Age Group

Children and Adolescents (Ages 3-18 Years)

• Perform RADT when clinical features suggest possible strep pharyngitis 2
• If RADT is positive: treat with antibiotics immediately (specificity ≥95% means false positives are rare) 1
• If RADT is negative: perform backup throat culture because RADT sensitivity is only 70-90%, meaning 10-30% of true infections are missed 1, 2
• The backup culture adds 18-24 hours to diagnosis but is essential in pediatric patients 2

Adults

• Perform RADT when clinical features suggest possible strep pharyngitis 2
• If RADT is positive: treat with antibiotics 1
• If RADT is negative: no backup culture is needed—proceed with symptomatic management for presumed viral pharyngitis 2

Children Under 3 Years

• Do not test routinely due to low incidence of Group A strep and rare occurrence of rheumatic fever 2
• Exception: test if risk factors present (e.g., older sibling with confirmed strep infection) 2

When NOT to Test

Do not test when overt viral features are present: 2

• Conjunctivitis
• Coryza (runny nose)
• Cough
• Diarrhea
• Hoarseness
• Discrete ulcerative stomatitis
• Viral exanthem (rash)

Throat Culture Details (When Needed)

If performing throat culture:

• Incubate plates at 35-37°C for 18-24 hours initially 1
• Re-examine plates at 48 hours if negative at 24 hours—additional incubation at room temperature identifies many additional positive cultures 1
• Use bacitracin disk test for presumptive identification: ≥95% of Group A strep show inhibition zones, while 83-97% of non-Group A strep do not 1
• The number of colonies on the plate cannot reliably distinguish true infection from carrier state 1, 2

Common Pitfalls to Avoid

• Do not treat based on clinical symptoms alone without laboratory confirmation—this leads to antibiotic overuse 2
• Do not test asymptomatic household contacts of patients with confirmed strep 2
• Do not use anti-streptococcal antibody titers for acute pharyngitis diagnosis—these reflect past, not current, infections 1, 2
• Do not perform routine post-treatment testing unless special circumstances exist 2
• Do not assume negative RADT in adults needs culture confirmation—the negative predictive value is sufficient for clinical decision-making 2

Clinical Decision Support

Use clinical scoring to determine who needs testing:

• Calculate Centor/McIsaac score (fever, tonsillar exudates, tender anterior cervical lymphadenopathy, absence of cough) 2
• 0-2 criteria: no testing needed 2
• 3-4 criteria: perform RADT 2

Test Performance Characteristics

• RADT specificity: ≥95% (positive results are highly reliable) 1, 2
• RADT sensitivity: 70-90% (misses 10-30% of infections) 1, 2
• Throat culture remains the gold standard with higher sensitivity than RADT 3, 4
• Newer optical immunoassay and chemiluminescent DNA probe tests may approach culture sensitivity, but require confirmatory data before routine use without backup culture 1