Testing for Streptococcal Carrier State
The best way to test for strep carriage is to obtain a throat culture during an asymptomatic interval—swabbing both tonsillar fossae and the posterior pharyngeal wall—but routine testing of asymptomatic individuals is not recommended unless specific high-risk circumstances exist. 1
When Testing Is Actually Indicated
Testing asymptomatic individuals for strep carriage should be reserved for specific high-risk scenarios only:
- Personal history of acute rheumatic fever – these patients require carrier identification and eradication to prevent recurrent rheumatic events 2, 3
- Documented community outbreaks of acute rheumatic fever or post-streptococcal glomerulonephritis 2, 3
- Outbreaks in closed/semi-closed settings such as military barracks, nursing homes, or schools with multiple cases 2, 3
- Close contact with invasive GAS disease (necrotizing fasciitis, toxic shock syndrome) – household contacts should be screened 3
- Multiple recurrent documented GAS pharyngitis in the index patient to identify household "ping-pong" transmission 3
Routine testing of asymptomatic household contacts of patients with ordinary strep pharyngitis is explicitly not recommended. 2, 3
Proper Specimen Collection Technique
When testing is indicated, proper technique is critical:
- Swab both tonsillar fossae (or tonsillar beds if tonsils removed) and the posterior pharyngeal wall – these are the only acceptable sites 4
- Avoid touching other oral areas before or after sampling the appropriate sites, as this causes false-negative results 4
- Do not compromise technique in uncooperative patients – an inadequate specimen is neither representative nor useful 4
- Avoid testing patients on antibiotics as this produces false-negative results 4
Culture Method for Carrier Detection
Standard throat culture remains the reference approach:
- Plate on blood agar and incubate at 35-37°C for 18-24 hours initially 4
- Re-examine negative plates at 48 hours after additional room temperature incubation, as this identifies a considerable number of additional positive results 4
- Anaerobic incubation and selective media may increase yield, though data are conflicting 4
- Bacitracin disk testing provides presumptive identification (≥95% of GAS show inhibition zones) 4
Limitations of Colony Count
The number of colonies on the culture plate cannot reliably distinguish carriers from acute infection:
- Carriers tend to have fewer colonies than patients with acute pharyngitis, but there is too much overlap for accurate differentiation on this basis alone 4, 1
- Clinical context matters more – consider the patient's age, season, local epidemiology (influenza/enteroviral activity), and presenting symptoms 1
Alternative Testing Approaches
- Serotyping of repeated isolates can help determine if the same strain persists (suggesting carriage) versus new acquisitions, but this requires specialized research laboratories and is not clinically practical 1
- Rapid antigen detection tests (RADTs) have excellent specificity (≥95%) but sensitivity of only 80-90%, and false-negatives often occur in carriers 4
- Nucleic acid amplification tests (NAATs) are more sensitive than culture but will detect even lower bacterial loads, potentially over-identifying carriers 5
Critical Pitfalls to Avoid
- Do not perform routine post-treatment cultures in asymptomatic patients after treating pharyngitis – this leads to inappropriate retreatment of carriers who develop intercurrent viral infections 1, 3
- Do not confuse asymptomatic positive cultures with active infection requiring treatment 2
- Remember that up to 20% of school-aged children are carriers during winter/spring, making universal testing impractical 1, 2, 3
- Carriers are unlikely to spread GAS to contacts and are at very low risk for complications, so identification serves no purpose in most clinical scenarios 1, 2