How Scarlet Fever is Diagnosed
Scarlet fever diagnosis requires throat culture or rapid antigen detection test (RADT) to confirm Group A Streptococcus (GAS) infection before starting antibiotics, combined with recognition of the characteristic clinical features including sandpaper-like rash, strawberry tongue, and pharyngitis. 1
Primary Diagnostic Approach
The diagnostic gold standard involves obtaining a throat swab from the posterior pharynx and tonsillar surfaces bilaterally for either throat culture or RADT. 1 Throat culture remains the reference standard for confirming GAS pharyngitis and provides definitive identification of Group A β-hemolytic streptococci. 1
Specimen Collection Technique
- Swab the posterior pharynx and tonsillar surfaces bilaterally using proper technique 1
- While nasopharyngeal specimens may be more effective for optimal pathogen detection, throat swabs remain the standard approach for GAS 1
- Collect specimens before initiating antibiotic therapy 1
Essential Clinical Features to Recognize
The diagnosis relies heavily on identifying the characteristic clinical presentation alongside laboratory confirmation:
Cardinal Features
- Sore throat that is often severe with pain on swallowing 2
- Fever ≥38°C (102°F) persisting for at least 5 days 1
- Distinctive sandpaper-like rash that typically begins on the trunk and spreads to extremities, with accentuation in the perineal region 1, 2
- "Strawberry tongue" - initially white-coated then bright red with prominent papillae 2
- Tonsillopharyngeal erythema with or without exudates 2
- Palatal petechiae 2
- Cervical lymphadenopathy, usually unilateral 1
Key Distinguishing Features
When differentiating scarlet fever from similar conditions:
- Unlike Kawasaki disease: Scarlet fever typically has exudative pharyngitis and responds rapidly to antibiotics 2
- Unlike viral pharyngitis: Scarlet fever rarely presents with cough, hoarseness, or conjunctivitis 2
- Unlike Rocky Mountain Spotted Fever: The rash in scarlet fever appears earlier in the illness, rarely becomes petechial, and does not typically involve palms/soles initially 1, 2
Epidemiological Context
- Most common in children aged 5-15 years 2
- Typically occurs in winter and early spring in temperate climates 2
- Transmitted through direct contact with saliva and nasal fluids of infected people 3
Critical Pitfalls to Avoid
Do not start antibiotics without laboratory confirmation unless the clinical presentation is classic and treatment cannot be delayed. 1 The combination of throat culture or RADT with clinical features prevents misdiagnosis and inappropriate antibiotic use.
Do not dismiss atypical presentations - scarlet fever can present with diverse symptoms and atypical rash distribution, including involvement limited to hands and feet in adults. 4 A high degree of suspicion is required for these rare presentations. 4
Early diagnosis is critical to reduce the risk of developing serious complications including acute rheumatic fever, glomerulonephritis, bacteremia, pneumonia, endocarditis, and meningitis. 3, 5