Why is a throat culture recommended for suspected strep (streptococcal) throat?

Why Get a Throat Culture for Strep

A throat culture is obtained to confirm the diagnosis of group A streptococcal pharyngitis when the rapid antigen detection test (RADT) is negative in children and adolescents, because RADTs miss 10-20% of true strep infections, and untreated strep can lead to acute rheumatic fever. 1

Primary Reasons for Throat Culture

Backup Confirmation After Negative RADT in Children

• In children and adolescents, a negative RADT must be confirmed with throat culture before withholding antibiotics because the sensitivity of rapid tests is only 80-90%, meaning false-negatives occur in truly infected patients 1
• The specificity of RADT is excellent (≥95%), so positive tests do not require culture confirmation—you can treat immediately with confidence 1
• Adults generally do not need backup cultures after negative RADT because they have only 5-10% prevalence of strep pharyngitis and extremely low risk of acute rheumatic fever 1

Gold Standard Diagnostic Test

• Throat culture on sheep blood agar remains the gold standard for documenting group A streptococci, with 90-95% sensitivity when performed correctly 1
• Culture results take 18-24 hours for initial reading, with additional overnight incubation at room temperature identifying more positive cases that would otherwise be missed 1

Clinical Implications of Missing Strep Diagnosis

Prevention of Serious Complications

• The primary justification for confirming strep pharyngitis is preventing acute rheumatic fever, which can occur if infection goes untreated 1
• Treatment can be safely delayed up to 9 days after symptom onset and still prevent rheumatic fever, so waiting for culture results does not compromise this goal 1
• Antibiotics do not prevent post-streptococcal glomerulonephritis, but do reduce peritonsillar abscess risk and limit spread during outbreaks 2

Avoiding Unnecessary Antibiotics

• Clinical features alone cannot reliably distinguish strep from viral pharyngitis—up to 70% of sore throat patients receive unnecessary antibiotics when diagnosis is based on symptoms alone 2, 3
• Only 20-30% of pharyngitis cases in children ages 5-15 are actually strep, meaning 70-80% are viral and don't need antibiotics 2

Proper Technique Matters

Specimen Collection

• The swab must touch both tonsillar surfaces (or fossae) and the posterior pharyngeal wall—other oral areas are unacceptable and compromise accuracy 1
• Poor swabbing technique from uncooperative children yields inadequate specimens that produce false-negative results 1
• Recent antibiotic use can cause false-negative cultures if the patient received antibiotics shortly before specimen collection 1

Culture Interpretation

• Plates should be incubated at 35-37°C for 18-24 hours, then re-examined at 48 hours to catch additional positive results 1
• The number of streptococcal colonies cannot reliably distinguish true infection from carrier state—there is too much overlap 1

Common Pitfalls to Avoid

• Do not skip backup culture in children with negative RADT—this is the most critical error, as you will miss 10-20% of true infections 1
• Do not perform follow-up cultures on asymptomatic patients after completing treatment—most positive results represent carrier state, not treatment failure 1
• Do not test or treat asymptomatic household contacts, even with recurrent infections in the family—up to one-third of households have asymptomatic carriers who don't need treatment 2
• Do not test children under 3 years old unless special risk factors exist (like an older sibling with confirmed strep), as strep pharyngitis and rheumatic fever are rare in this age group 1, 3