On which day since the onset of fever should a dengue (Dengue Virus) blood test be performed?

Timing of Dengue Blood Tests After Fever Onset

Dengue blood tests should be performed based on the timing of symptom onset, with nucleic acid amplification tests (NAATs) or NS1 antigen detection within the first 7 days of fever, and IgM antibody testing from day 3-5 onward, with optimal serologic testing at 7-10 days after fever onset.

Diagnostic Testing Timeline

Early Phase (≤7 days after fever onset)

• NAAT (PCR) testing: Most effective when performed on serum collected within 7 days of symptom onset 1
• NS1 antigen detection: Can be detected as early as day 1 after symptom onset and remains detectable up to 10 days 1
• Viremia period: Patients are typically viremic for 4-6 days after symptom onset 1

Later Phase (>7 days after fever onset)

• IgM antibody testing: Becomes detectable 3-5 days after symptom onset and remains positive for 2-3 months 1
• IgG antibody testing: Appears 10-12 days after symptom onset and remains detectable for months to years 1
• Optimal serologic sampling: A serum specimen collected 7-10 days after disease onset is recommended for definitive serologic diagnosis 1

Testing Strategy Based on Time Since Fever Onset

• Days 1-7:

  • Primary approach: NAAT or NS1 antigen detection
  • These tests provide direct evidence of viral presence 1
  • RT-PCR can detect virus up to day 7-8 of fever, even after defervescence 2

• After Day 7:

  • Primary approach: IgM and IgG antibody testing
  • Serologic testing becomes increasingly important as viral components decrease 1
  • By day 8 of fever, serological diagnosis can be made in up to 100% of patients using appropriate ELISA tests 2

Special Considerations

• Secondary infections: IgM antibodies may not be detectable in secondary dengue infections 1

  • In these cases, a positive NS1 antigen + positive IgG suggests secondary infection 3

• Cross-reactivity: Antibody tests may show false positives due to prior flavivirus infections (e.g., West Nile virus, Zika virus) 1

  • Plaque reduction neutralization tests (PRNTs) may be needed for confirmation in ambiguous cases 1

• Pregnant women: For symptomatic pregnant women, collect serum and urine specimens as soon as possible within 12 weeks of symptom onset for concurrent dengue and Zika virus testing 1

Test Performance

• Viral isolation yields positive results on blood collected before the 5th day of fever 2
• Half of serum samples collected on day 3 after fever onset may be positive for dengue antibodies, with positivity rates increasing to 100% by day 6 4
• Whole blood samples may show prolonged detection of dengue virus RNA compared to plasma, sometimes remaining positive for weeks to months after infection 5

Common Pitfalls to Avoid

• Waiting too long for NAAT/NS1 testing: These tests are most effective in the first week of illness
• Testing too early for antibodies: IgM may not be detectable until 3-5 days after symptom onset
• Missing the diagnosis in secondary infections: IgM may be absent; consider NS1 and IgG testing
• Not repeating negative tests: An initially negative serologic test should be followed up with repeat testing on a specimen collected 7-10 days after symptom onset if dengue is strongly suspected 1

By following this testing timeline, clinicians can optimize the chances of accurately diagnosing dengue infection while minimizing false negative results due to inappropriate test timing.