Which laboratory test should be ordered for a throat swab to detect Group A Streptococcus (strep throat)?

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Last updated: September 28, 2026 • View editorial policy

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Laboratory Testing for Strep Throat

Order a rapid antigen detection test (RADT) for Group A Streptococcus, and if negative in children/adolescents, follow with a throat culture on blood agar plate. 1

Testing Algorithm

For Children and Adolescents (≥3 years old):

  1. First-line: Rapid Antigen Detection Test (RADT)

    • Highly specific (~95%) - positive results can be trusted 1
    • Sensitivity 70-90% - negative results need confirmation 1
    • Results available within minutes

  2. If RADT is negative: Order throat culture

    • Culture on blood agar plate at 35°C-37°C for 18-24 hours 1
    • Re-examine plates at 48 hours if initially negative 1
    • This backup culture is essential in pediatric patients due to the 20-30% prevalence of GAS pharyngitis and risk of rheumatic fever 1

For Adults:

  • RADT alone is sufficient - backup throat culture is not necessary for negative results under usual circumstances 1
  • The lower prevalence of GAS pharyngitis and minimal risk of rheumatic fever in adults justifies this simplified approach 1

Specific Test Orders

According to IDSA/ASM guidelines, you can order 2:

  • Rapid antigen test for Streptococcus pyogenes (point-of-care or lab-based)
  • Throat culture (pharyngeal swab in swab transport device, room temperature, 2-hour transport time)
  • Nucleic acid amplification test (NAAT) - increasingly available and more sensitive than RADTs 2

Emerging Option: Molecular Testing

NAATs for GAS are superior to both RADTs and culture in terms of sensitivity and speed, with recent studies showing 94-100% sensitivity 3, 4. These tests:

  • Do not require backup culture when negative 2
  • Provide results in hours rather than 24-48 hours
  • Detect more true positives than culture 3

However, NAATs are not yet universally available and cost more than traditional methods 4.

Critical Caveats

Do NOT test patients with clear viral features: cough, rhinorrhea, hoarseness, or oral ulcers strongly suggest viral etiology 1. Testing in these cases leads to unnecessary antibiotic use.

Do NOT test children <3 years old - acute rheumatic fever is rare in this age group 1

Practical Specimen Collection

  • Use a dual swab technique in pediatric patients when starting with RADT - if the first swab is positive, discard the second; if negative, send the second for culture 2
  • Swab the posterior pharynx and tonsillar areas
  • Transport in appropriate swab transport device at room temperature within 2 hours 2

Important Limitation

Neither culture nor RADTs distinguish between acute GAS infection and asymptomatic carriage with concurrent viral pharyngitis 1. However, they successfully identify the majority of patients who do NOT need antibiotics - critically important since 70% of sore throat patients receive antibiotics while only 20-30% actually have GAS 1.

References

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.