Scarlet Fever Diagnosis
Scarlet fever is diagnosed clinically by the characteristic triad of fever, pharyngitis, and a distinctive sandpaper-like rash, but microbiological confirmation with throat culture or rapid antigen detection test (RADT) is essential to confirm Group A Streptococcus (GAS) infection and guide antibiotic treatment. 1, 2
Key Clinical Features
Classic Presentation
- Sudden onset sore throat with pain on swallowing, typically appearing before the rash 1, 3
- High fever is a hallmark feature 1, 3
- Characteristic sandpaper-like, papular rash that typically appears 1-2 days after onset of other symptoms (median 1-day delay) 4, 5
- Tonsillopharyngeal erythema with or without patchy exudates 1, 3
- Tender, enlarged anterior cervical lymph nodes 1, 3
- Palatal petechiae ("doughnut lesions") - highly suggestive when present 1, 3
- Beefy red, swollen uvula 1, 3
Associated Systemic Symptoms
- Headache is commonly reported 3
- Nausea, vomiting, and abdominal pain, especially in younger children 1, 3
Epidemiological Clues
- Peak age 5-15 years, though can occur at any age 1, 3, 6
- Winter or early spring presentation in temperate climates 1, 3
- History of exposure to documented streptococcal case 1, 3
Features Suggesting Viral (Not Scarlet Fever)
These findings argue strongly against scarlet fever and should prompt consideration of viral etiology without GAS testing: 1, 2
- Conjunctivitis 1, 2, 3
- Coryza (nasal discharge) 1, 2, 3
- Hoarseness 1, 2, 3
- Cough 1, 2
- Discrete oral ulcers 2, 3
Diagnostic Testing Algorithm
When to Test
- Perform microbiological testing when clinical and epidemiological features suggest GAS pharyngitis (sudden sore throat, fever, absence of viral features) 1, 2
- Do NOT test when obvious viral features are present (cough, rhinorrhea, conjunctivitis, hoarseness) - the pretest probability is too low 1, 2
Testing Methods
- Throat culture remains the gold standard with vigorous swabbing of both tonsils and posterior pharynx 1
- RADT has 90-96% specificity and 79-88% sensitivity - a positive test is diagnostic and treatment should be initiated immediately 2, 7
- In children and adolescents, negative RADT must be confirmed with throat culture due to lower sensitivity 2
- In adults, clinical algorithm without microbiological confirmation is NOT recommended as it leads to inappropriate antibiotic overuse 1
Critical Diagnostic Pitfalls
Delayed Recognition
- Rash appears AFTER other symptoms in 71% of cases, with median 1-day delay - do not wait for rash to consider scarlet fever 5
- In 28% of cases, scarlet fever is not considered at first consultation, with symptoms frequently misattributed to viral infection 5
- Older children (≥5 years) presenting with sore throat have higher rates of delayed diagnosis (OR 2.8) because this presentation mimics viral pharyngitis 5
- Atypical rash distribution (e.g., only hands and feet) can lead to missed diagnosis 6
Cannot Distinguish Infection from Carriage
- Positive throat culture or RADT may reflect chronic GAS colonization (up to 15% of school-age children in winter/spring) with concurrent viral pharyngitis 1
- Quantitation of GAS cannot differentiate - even sparse growth may represent true infection 1
- Clinical judgment must integrate microbiological results with clinical presentation 1
Laboratory Testing Limitations
- Total leukocyte count and differential cannot reliably distinguish bacterial from viral pharyngitis and should not guide antibiotic decisions 2
- Even with all clinical features suggestive of GAS, only 35-50% are confirmed positive on testing 2
Age-Specific Considerations
- Children <3 years may have less apparent classic findings 3
- Infants may present with excoriated nares or purulent nasal discharge rather than typical pharyngitis 3
- Older children (≥5 years) more commonly present with sore throat at onset (OR 3.1) 5
Importance of Early Diagnosis
Immediate antibiotic treatment upon positive microbiological confirmation is essential to: 4, 7