As a VP (Venereal/Visiting Physician or medical practitioner), the timing of dengue tests is important because different tests become positive at different stages of the illness.
Dengue NS1 Antigen Test
- Best performed during the early phase of dengue infection.
- Usually positive from Day 1 to Day 5–7 after the onset of fever.
- Highest sensitivity during the first few days of illness.
- A positive NS1 test strongly suggests an acute dengue infection.
Dengue Antibody Tests
IgM Antibody
- Usually becomes detectable around Day 4–5 of illness.
- Peaks at about 2 weeks.
- Can remain detectable for 2–3 months or longer.
- A positive IgM generally indicates a recent dengue infection.
IgG Antibody
- In a first dengue infection, IgG usually appears after Day 7–10.
- In a second (secondary) dengue infection, IgG may rise much earlier, sometimes within the first few days of fever.
- IgG can remain positive for many years and indicates past exposure to dengue virus.
Practical Approach
| Day of Illness | Preferred Test |
|---|---|
| Day 1–5 | NS1 Antigen (± PCR if available) |
| Day 5–7 | NS1 + IgM |
| After Day 7 | IgM and IgG antibodies |
| >2 weeks | IgM and IgG (NS1 often becomes negative) |
Example
If a patient develops fever on Monday:
- Tuesday–Friday: NS1 antigen is the most useful test.
- Saturday onward: IgM testing becomes increasingly reliable.
- The following week: IgM and IgG antibody testing are more informative than NS1.
Keep in mind that test performance can vary depending on whether it is a first dengue infection or a repeat infection, and clinical assessment plus platelet count, hematocrit, and other laboratory findings remain important in diagnosis. For suspected severe dengue, management should not be delayed while waiting for test results.