Can a current dengue infection and a past infection from a year ago result in a positive antigen and Immunoglobulin G (IgG) test?

Dengue Diagnostic Interpretation: Current and Past Infections

Yes, it is possible to have both a positive dengue antigen and IgG if you currently have dengue infection and also had it a year ago. 1, 2

Understanding Dengue Diagnostic Markers

The interpretation of dengue diagnostic tests depends on the timing of the specimen collection and the patient's infection history:

Acute Phase Markers (≤7 days after symptom onset):

• NS1 Antigen: Detectable from day 1 of fever onset 2
• Viral RNA: Detectable by NAAT (nucleic acid amplification test) 1
• IgM antibodies: Begin to appear 3-5 days after onset 1
• IgG antibodies:
  • In primary infections: Appear later, usually after day 7
  • In secondary infections: Can rise rapidly within 1-2 days 1, 2

Convalescent Phase Markers (>7 days after symptom onset):

• IgM antibodies: Peak around 2 weeks and can persist for 2-3 months 1
• IgG antibodies: Rise more slowly in primary infections, rapidly in secondary infections 1

Interpretation of Your Scenario

In your specific scenario (current infection + previous infection from a year ago):

1. Positive NS1 antigen: Indicates current active dengue virus replication 2

2. Positive IgG: Can be explained by:

   • Previous dengue infection from a year ago (IgG can persist for years) 1
   • Early secondary immune response if this is indeed a second infection 1, 3

This pattern (positive antigen + positive IgG) is consistent with a secondary dengue infection in someone previously exposed to dengue virus 1, 4.

Clinical Implications

The presence of both markers has important clinical implications:

• Increased risk of severe disease: Secondary dengue infections are associated with higher risk of severe manifestations 4
• Triple positivity warning: If NS1, IgM, and IgG are all positive, this pattern has been associated with increased risk of severe thrombocytopenia 4
• Monitoring requirements: Patients with secondary infections require closer monitoring for warning signs of severe dengue 2

Diagnostic Algorithm

To confirm this interpretation:

1. If within 7 days of symptom onset:

   • Positive dengue NS1 antigen + positive IgG = likely secondary dengue infection 1
   • Consider NAAT testing to confirm active infection if available 1

2. If beyond 7 days of symptom onset:

   • Test for both IgM and IgG antibodies 1
   • Consider PRNT (plaque reduction neutralization test) if available to confirm specific flavivirus infection 1

Common Pitfalls to Avoid

• Cross-reactivity: IgG antibodies may cross-react with other flaviviruses (like Zika, Japanese encephalitis) 1, 5
• Misinterpreting timing: The presence of IgG alone doesn't distinguish between current and past infections without additional markers 1
• Overlooking severity risk: Secondary dengue infections can progress more rapidly to severe disease 2, 4

Remember that while a positive antigen indicates current infection, the presence of IgG without considering other markers and clinical context cannot definitively distinguish between primary and secondary dengue infections 6.