Laboratory Evaluation for Fever Followed by Rash
Order a complete blood count (CBC) with differential, comprehensive metabolic panel, blood culture, and acute serology for tickborne rickettsial diseases (Rocky Mountain Spotted Fever, ehrlichiosis, anaplasmosis) as the essential initial workup, but do not delay empiric treatment while awaiting results. 1
Core Laboratory Tests
The following tests should be ordered immediately for any patient presenting with fever followed by rash:
CBC with differential: Essential to identify thrombocytopenia (common in RMSF and ehrlichiosis), leukopenia (suggestive of ehrlichiosis/anaplasmosis in up to 53% of cases), and increased band forms 1
Comprehensive metabolic panel: Detects hyponatremia and elevated hepatic transaminases (frequently seen in RMSF), and modest liver enzyme elevations particularly suggestive of ehrlichiosis 1
Blood cultures: Critical to rule out life-threatening bacterial infections including meningococcemia, which can present identically to tickborne diseases 1
Rapid Streptococcus pharyngitis screen: Helps exclude streptococcal disease with exanthem from the differential 1
Specialized Serologic and Molecular Testing
Acute serum for IgG and IgM antibodies: Obtain for Rickettsia rickettsii, Ehrlichia chaffeensis, and Anaplasma phagocytophilum, but do not wait for results before initiating treatment 1
PCR testing: If available from reference laboratories, order PCR for E. chaffeensis and A. phagocytophilum using EDTA whole blood 1
Convalescent-phase serology: Must be performed 2-4 weeks after acute illness to confirm diagnosis 1
Critical Interpretation Points
Key Laboratory Findings by Disease
Rocky Mountain Spotted Fever:
- Normal WBC count is typical (frequently observed in viral infections and RMSF) 1
- Thrombocytopenia develops as disease progresses, though normal platelet count is common early in illness 1
- Hyponatremia and mild hepatic transaminase elevations 1
Ehrlichiosis/Anaplasmosis:
- Leukopenia in up to 53% of patients 1
- Thrombocytopenia in up to 94% of patients 1
- Modest elevations of liver transaminases 1
- Peripheral blood smear may show morulae (only 1-20% sensitive for HME) 1
Timing Considerations
The timing of rash relative to fever onset is diagnostically critical:
- Rubella: Rash appears -1 day to +2 days from fever onset; if rash appears ≥3 days after fever, rubella is unlikely 2
- Tickborne diseases: Rash characteristics, distribution pattern, and progression timing relative to fever provide essential diagnostic clues 1
Additional Contextual Testing
Based on clinical presentation, consider:
- Urinalysis: Sterile pyuria may occur and should not be mistaken for partially treated urinary tract infection 3
- CSF analysis (if neurologic symptoms present): Typically shows pleocytosis (<100 cells/μL), moderately elevated protein (100-200 mg/dL), and normal glucose in RMSF 1
Common Pitfalls to Avoid
Never delay empiric antibiotic therapy while waiting for serologic results, as early treatment is critical for preventing morbidity and mortality from tickborne diseases 1
Do not rule out tickborne disease based on absence of tick bite history—many patients do not recall tick exposure 1
Consider empiric ceftriaxone in addition to doxycycline when meningococcemia cannot be reliably distinguished from tickborne disease on clinical grounds alone 1
Recognize that serologic tests are insensitive early in illness—negative acute serology does not exclude diagnosis 1
Essential Historical Information
Query the patient about: