What are the recommended tests for evaluating scrub typhus?

Recommended Diagnostic Tests for Scrub Typhus

The optimal diagnostic approach for scrub typhus includes PCR testing of blood samples during early disease (within 7 days of symptom onset) and IgM antibody detection by ELISA or rapid diagnostic tests for patients presenting later in the disease course.

Diagnostic Testing Algorithm

Early Disease (≤7 days of symptoms)

• First-line test: Quantitative real-time PCR (qPCR) targeting the 47kDa gene

  • Highest sensitivity (97%) and perfect specificity 1
  • Most reliable for early diagnosis before antibody development

• Alternative molecular tests:

  • Loop-mediated isothermal amplification (LAMP) assay (91.7% sensitivity, 77.2% specificity) 1
  • Conventional PCR targeting the 56kDa gene or groEL gene 2

Later Disease (>7 days of symptoms)

• First-line tests: Serological methods
  • IgM ELISA (92% sensitivity, 94% specificity) 1
  • Immunochromatographic rapid diagnostic test (RDT) (92% sensitivity, 92% specificity) 1

Special Considerations

• Eschar sampling: When present, PCR testing of eschar material provides excellent diagnostic yield 3
• Blood cultures: Should be obtained when patients present with signs of septicemia 3

Specimen Collection and Handling

• Blood specimens: EDTA-anticoagulated whole blood (3-5 mL preferred volume) 3
• Eschar specimens: Swab of unroofed eschar or 4mm punch biopsy of eschar 3
• Timing: Collect specimens before or within 48 hours of starting doxycycline therapy 3

Test Performance Characteristics

Molecular Methods

• qPCR: Most sensitive during first week of illness (97% sensitivity, 100% specificity) 1
• LAMP: Good alternative when qPCR unavailable (91.7% sensitivity, 77.2% specificity) 1
• groEL PCR: Can detect bacterial DNA even in cases with up to 10 days of fever 2

Serological Methods

• IgM ELISA: Excellent for later-stage diagnosis (92% sensitivity, 94% specificity) 1
• RDT: Good point-of-care option (92% sensitivity, 92% specificity) 1
• IFA: Traditional reference standard but suboptimal specificity (95% sensitivity, 74% specificity) 1
• Weil-Felix test: Poor performance (43.5% sensitivity) and not recommended 4

Pitfalls and Limitations

• Cross-reactivity: Serological tests may show false positives (8% rate) with other acute febrile illnesses including dengue, leptospirosis, and spotted fever due to nonspecific binding of IgM 1

• Timing challenges:

  • PCR sensitivity diminishes after 7 days of illness 3
  • Antibody tests may be negative in early disease 3

• Treatment effect:

  • PCR sensitivity decreases within 48 hours of doxycycline therapy 3

• Misdiagnosis risk:

  • Clinical presentation overlaps with other tropical infections 5
  • Absence of pathognomonic eschar in many cases necessitates laboratory confirmation 5

Recommended Approach

1. Determine symptom duration
2. For early disease (≤7 days): Prioritize qPCR testing
3. For later disease (>7 days): Use IgM ELISA or RDT
4. Consider combined approach: Using both molecular and serological methods increases sensitivity with minimal reduction in specificity 6

By following this diagnostic algorithm, clinicians can optimize the detection of scrub typhus, enabling prompt treatment and reducing morbidity and mortality associated with delayed diagnosis.