What are the recommended tests for diagnosing Ehrlichiosis?

Diagnostic Testing for Ehrlichiosis

The recommended diagnostic approach for ehrlichiosis includes PCR of whole blood during the acute phase of illness, paired serologic testing using indirect immunofluorescence antibody (IFA) assays, and examination of blood smears for morulae in leukocytes. 1

Primary Diagnostic Tests

1. Polymerase Chain Reaction (PCR)

• Most useful during the acute phase of illness before antibiotics are administered
• Provides rapid detection of Ehrlichia DNA in whole blood samples
• Sensitivity ranges from 52-87% for E. chaffeensis detection 1
• Important considerations:
  • Collect blood BEFORE antibiotic administration as doxycycline can decrease PCR sensitivity 1
  • PCR offers definitive diagnosis with genus-specific and species-specific primers 2

2. Serologic Testing

• Indirect Immunofluorescence Antibody (IFA) assay is the gold standard for serologic confirmation 1

  • Requires paired acute and convalescent sera collected 2-4 weeks apart
  • A fourfold or greater increase in IgG antibody titer confirms the diagnosis
  • Single elevated titer (≥64) supports but does not confirm diagnosis 1

• Timing of serologic testing is critical:

  • First week of illness: Only 22% of patients have detectable antibodies 3
  • Second week: 68% have detectable antibodies 3
  • Third week: Nearly 100% have detectable antibodies 3
  • IFA is 94-100% sensitive after 14 days of illness 1

3. Blood Smear Examination

• Microscopic examination of Wright-Giemsa stained blood smears
• Look for morulae (clusters of bacteria) in the cytoplasm of infected leukocytes
• Detection rates:
  • 1-20% of patients with human monocytic ehrlichiosis (HME) 1
  • 20-80% of patients with human granulocytic anaplasmosis (HGA) 1
• Should be performed by an experienced microscopist
• Negative result should not rule out diagnosis 1

Diagnostic Algorithm

1. Acute phase (first week of illness):

   • Collect whole blood for PCR BEFORE antibiotic administration
   • Examine blood smears for morulae
   • Collect acute serum sample for baseline IFA testing
   • Begin appropriate treatment without waiting for results

2. Convalescent phase (2-4 weeks later):

   • Collect convalescent serum sample
   • Compare paired sera for fourfold rise in antibody titer

Important Considerations and Pitfalls

• Cross-reactivity: Antibodies against Ehrlichia species may cross-react with other rickettsial pathogens 1, 4

  • 36.5% of confirmed ehrlichiosis patients may have antibody titers considered diagnostic for other rickettsial infections 3
  • Test for multiple pathogens when ehrlichiosis is suspected

• Timing pitfalls:

  • Early serologic testing (first week) is often negative
  • Antibiotic therapy may diminish development of convalescent antibodies 1

• Interpretation challenges:

  • A single elevated antibody titer is never sufficient to confirm acute infection 1
  • Antibodies may persist for months to years after infection 1

• Laboratory selection:

  • No standardized antigens, conjugates, or threshold values exist among laboratories 1
  • Consult individual laboratories regarding their test threshold levels

By following this diagnostic approach, clinicians can effectively diagnose ehrlichiosis and initiate appropriate treatment to reduce morbidity and mortality associated with this tickborne disease.