What is the recommended initial test for a suspected strep (streptococcal) carrier?

Testing for Strep Carrier Status

The recommended initial test for a suspected strep carrier is a throat culture on a sheep blood agar plate, which remains the gold standard for detecting group A streptococcal colonization. 1

Diagnostic Approach for Strep Carriers

Primary Testing Method

• Throat culture on sheep blood agar plate:
  • Sensitivity of 90-95% when performed correctly 1
  • Remains the standard for documenting the presence of group A streptococci
  • Should be incubated at 35-37°C for 18-24 hours, with additional examination at 48 hours for initially negative cultures 1

Proper Specimen Collection

• Swab should be obtained from:
  • Surface of both tonsils (or tonsillar fossae)
  • Posterior pharyngeal wall
• Avoid touching other areas of the oropharynx or mouth before or after sampling the appropriate areas 1

Rapid Antigen Detection Tests (RADTs)

• Not recommended as the initial test for carriers due to:
  • Lower sensitivity (80-90%) compared to throat culture 1
  • False negatives may occur in true carriers
  • A negative RADT should always be confirmed with a conventional throat culture 1

Special Considerations for Strep Carriers

When to Test for Carrier Status

Testing for carrier status is warranted in specific situations:

• When "Ping-Pong" spread of group A streptococci is occurring within a family 1
• During outbreaks of acute rheumatic fever or poststreptococcal glomerulonephritis 1
• During outbreaks in closed or semi-closed communities 1
• For patients with histories of rheumatic fever 1

Distinguishing Carriers from Active Infection

• Antibody titers to streptococci can be helpful in epidemiological studies to separate carriers from those with acute infection 1
• These titers reflect past rather than present immunologic events and are not useful for routine diagnosis 1

Common Pitfalls to Avoid

1. Relying solely on RADTs: While convenient, their lower sensitivity makes them inadequate for detecting carriers 1, 2

2. Improper specimen collection: Inadequate sampling can lead to false-negative results 1

3. Insufficient incubation time: Cultures should be examined at both 24 and 48 hours 1

4. Testing after antibiotic exposure: Recent antibiotic use can lead to false-negative results 1

5. Unnecessary follow-up testing: Routine follow-up cultures are not indicated for asymptomatic patients who have completed appropriate therapy 1

Management Implications

• Most asymptomatic patients with group A streptococci after completing therapy are carriers rather than having active infection 1
• Carriers generally do not require treatment unless they fall into specific risk categories 1
• The clinical significance of the number of colonies on culture cannot reliably differentiate carriers from active infection 1

By following this evidence-based approach to testing for strep carriers, clinicians can appropriately identify and manage patients with streptococcal colonization while avoiding unnecessary testing and treatment.