Investigations for Suspected Scarlet Fever in a 5-Year-Old
Confirm the diagnosis with a rapid antigen detection test (RADT) and/or throat culture before initiating antibiotics, unless the child appears severely ill. 1
Immediate Diagnostic Testing
Primary Microbiologic Confirmation
- Obtain a throat swab for rapid antigen detection test (RADT) as the first-line diagnostic approach for suspected Group A streptococcal infection 1
- A positive RADT result is diagnostic and sufficient to confirm scarlet fever without additional testing 1
- If the RADT is negative, perform a backup throat culture in this 5-year-old child, as backup cultures are mandatory in pediatric patients due to the risk of rheumatic fever 1
- The backup culture requirement in children distinguishes pediatric from adult management, where cultures are generally unnecessary after negative RADT 1
Laboratory Studies to Obtain
- Complete blood count (CBC) with differential to assess for leukocytosis, bandemia, or other abnormalities 1
- Comprehensive metabolic panel to evaluate for complications if the child appears systemically ill 1
- Blood culture if the patient has high fever (≥105°F/40.6°C) or appears toxic, to rule out bacteremia or other serious bacterial infections 1
Critical Differential Diagnoses to Exclude
Life-Threatening Conditions Requiring Urgent Evaluation
- Rule out meningococcemia urgently if petechiae or purpura develop, as this cannot be reliably distinguished from scarlet fever on clinical grounds alone 1, 2
- Consider Rocky Mountain Spotted Fever (RMSF) if the rash involves palms and soles, particularly during tick season (spring/summer), even without tick bite history 1, 2
- Evaluate for tickborne rickettsial disease if the child has been in tick-infested areas, as up to 40% of patients don't recall tick exposure 2
Additional Diagnostic Considerations
- Obtain acute serology for R. rickettsii, E. chaffeensis, and A. phagocytophilum if rickettsial disease is suspected, though results won't be available for 3+ days and shouldn't delay treatment 1, 2
- Consider viral causes (enterovirus, adenovirus, EBV) if clinical features suggest viral etiology such as cough, rhinorrhea, or conjunctivitis 1
Testing NOT Recommended
- Do not order antistreptococcal antibody titers (ASO, anti-DNase B) for acute diagnosis, as these indicate past infection and are not useful for diagnosing acute pharyngitis 1
- Avoid diagnostic testing if viral features predominate (cough, rhinorrhea, hoarseness, oral ulcers), as these strongly suggest viral rather than streptococcal etiology 1
- Do not test household contacts routinely unless they are symptomatic 1
- Do not perform follow-up post-treatment testing routinely after completing antibiotics 1
Key Clinical Features to Document
History Elements
- Duration of fever and rash onset relative to fever 1, 2
- Recent outdoor activities or tick exposure, even if not recalled 2
- Sick contacts or known streptococcal exposures 1
- Medication use to rule out drug reactions 1
Physical Examination Findings
- Characteristic scarlet fever features: sandpaper-like rash, strawberry tongue, tonsillopharyngeal erythema with exudates, palatal petechiae, tender anterior cervical lymphadenopathy 1, 3, 4
- Rash distribution: typically spares palms and soles in scarlet fever (unlike RMSF) 1, 2
- Check for attached ticks on full body examination 2
- Assess for petechiae or purpura suggesting more serious infection 1, 2
Common Pitfalls to Avoid
- Never delay appropriate antibiotic therapy while awaiting culture results if RADT is positive or if the child appears severely ill 1, 5
- Do not dismiss rickettsial disease based on normal WBC count, as normal white blood cell counts are frequently observed early in RMSF 1, 2
- Thrombocytopenia (platelet count 50 × 10⁹/L) raises concern for rickettsial disease rather than uncomplicated scarlet fever 1, 2
- If rickettsial disease cannot be excluded and the child appears ill, initiate doxycycline empirically (2.2 mg/kg twice daily) regardless of age, as delaying treatment can be fatal 2, 6