Laboratory Testing for Unstable Eschar
For a patient with unstable eschar, you should prioritize obtaining a punch biopsy (≥4 mm) of the eschar for PCR testing, along with EDTA-anticoagulated whole blood (3-5 mL) for PCR, as eschars represent the optimal clinical specimen for diagnosing spotted fever group rickettsial infections when present. 1
Primary Diagnostic Approach
The clinical context of "unstable eschar" most commonly suggests a tickborne rickettsial disease, particularly spotted fever group (SFG) rickettsioses, where eschars are pathognomonic lesions containing abundant organisms. 1
Eschar-Specific Testing (Highest Yield)
- Fresh tissue punch biopsy (≥4 mm) of the eschar is the single best specimen, as eschars contain abundant numbers of SFG rickettsiae relative to blood 1
- PCR testing of the eschar biopsy should be performed immediately, as it is most sensitive during the first week of illness and before or within 48 hours of beginning doxycycline therapy 1
- Culture of the eschar tissue can be attempted if available at reference centers, though this requires BSL-3 facilities and confirmation may require ≥10 days 1
- Swab of unroofed eschar is a less invasive alternative to biopsy for PCR, though it is less sensitive and does not allow for culture or immunohistochemistry 1
Blood Testing (Essential Concurrent Testing)
- EDTA-anticoagulated whole blood (3-5 mL) for PCR testing of SFG rickettsiae, Anaplasma, and Ehrlichia species 1
- Blood smear (Wright or Giemsa stain) can provide rapid presumptive diagnosis for Anaplasma and Ehrlichia, though sensitivity is low and requires an experienced microscopist 1
- Acute serum (3-5 mL) for indirect fluorescent antibody (IFA) testing, though antibodies are often absent during the first week of illness 1
Standard Laboratory Panel
- Complete blood count (CBC) with differential to assess for leukocyte abnormalities and anemia 1, 2
- Comprehensive metabolic panel including creatinine, blood urea nitrogen (BUN), electrolytes (sodium, potassium), and liver function tests 1
- Convalescent serum (2-4 weeks later) for IFA to confirm diagnosis through fourfold or greater increase in titer between acute and convalescent specimens 1
Critical Timing Considerations
All specimens must be collected before initiating doxycycline therapy or within 48 hours of starting treatment, as sensitivity of PCR and culture diminishes rapidly after antibiotic administration. 1 This is a critical pitfall to avoid—do not delay specimen collection while awaiting test results or specialist consultation.
Transport and Handling Requirements
- Fresh tissue biopsies should be transported on ice and processed within 1 hour 1
- EDTA-anticoagulated blood should be maintained at room temperature and processed within 1 hour 1
- Serum samples can be transported at room temperature within 2 hours 1
- Formalin-fixed tissue specimens can be transported at room temperature within 2 hours for later immunohistochemistry if needed 1
Common Pitfalls to Avoid
Do not rely solely on serology during acute presentation, as antibodies are typically absent during the first week of illness when patients present with eschar. 1 The eschar itself provides the highest diagnostic yield through direct pathogen detection via PCR or culture.
Do not use serum for PCR as the primary specimen when an eschar is present—molecular assessment of serum is generally less sensitive than evaluation of whole blood or eschar tissue. 1
Avoid delaying empiric doxycycline therapy while awaiting laboratory confirmation, but ensure all specimens are collected first. 1