What tests are used to diagnose scarlet fever?

Diagnostic Tests for Scarlet Fever

Scarlet fever is primarily a clinical diagnosis based on characteristic features, but throat culture or rapid antigen detection test (RADT) should be performed to confirm Group A Streptococcus (GAS) infection before initiating antibiotic therapy. 1

Primary Diagnostic Approach

Clinical Diagnosis

The diagnosis begins with recognizing the classic presentation:

• Fever ≥38°C (102°F) 2
• Distinctive sandpaper-like rash that typically starts on the trunk and spreads to extremities 3, 4
• Sore throat with tonsillopharyngitis 3, 4
• Oral changes including "strawberry tongue" or "raspberry tongue" 5
• Cervical lymphadenopathy 2

Laboratory Confirmation

Throat Culture (Gold Standard)

• Throat culture remains the reference standard for confirming GAS pharyngitis, with approximately 19% positivity rate in patients with sore throat 6
• Specimens should be obtained from the posterior pharynx and tonsillar surfaces 1
• Culture provides definitive identification of Group A β-hemolytic streptococci 1

Rapid Antigen Detection Test (RADT)

• RADT offers immediate results with 91% sensitivity and 91% specificity compared to throat culture 6
• The test has a positive predictive value of 73% and negative predictive value of 98% 6
• Sensitivity increases to 97% in patients with three or four clinical features (fever, lack of cough, tonsillar exudate, cervical lymphadenopathy) 6
• A positive RADT allows immediate treatment initiation without waiting for culture results 6

Diagnostic Algorithm

Step 1: Clinical Assessment

• Document presence of fever (≥38°C), characteristic rash, pharyngitis, and oral changes 2, 4
• Note the presence of tonsillar exudate and anterior cervical lymphadenopathy 6

Step 2: Laboratory Testing

• Perform RADT immediately if available in your practice setting 6
• If RADT is positive, initiate antibiotic therapy immediately without culture confirmation 6
• If RADT is negative but clinical suspicion remains high, obtain throat culture for confirmation 1, 6

Step 3: Confirmatory Testing

• Antistreptolysin O (ASO) titer can be performed in patients >11 years if diagnosis remains uncertain, though this is not routinely necessary 6
• ASO titers are retrospective and not useful for acute diagnosis 6

Important Clinical Caveats

When to Suspect Atypical Presentation

• Be alert for atypical rash distribution involving only hands and feet, which can occur in adults and may be misdiagnosed 3
• Scarlet fever can affect any age group, not just children, requiring high clinical suspicion in adults 3

Testing Pitfalls to Avoid

• Never delay antibiotic treatment while awaiting culture results if clinical presentation is classic and RADT is positive 7
• Do not perform routine post-treatment throat cultures unless symptoms persist or recur 2
• Throat cultures for asymptomatic household contacts are not routinely indicated except in outbreak situations 1, 2

Outbreak Management

• In documented outbreaks (schools, day care centers), perform throat cultures on all patients but treat only those with positive results 1
• This targeted approach prevents unnecessary antibiotic use while controlling spread 1

Specimen Collection Considerations

• Nasopharyngeal specimens are more effective than throat swabs for optimal pathogen detection, though throat swabs remain standard for GAS 1
• Proper technique involves swabbing the posterior pharynx and tonsillar surfaces bilaterally 1