Dengue NS1 Antigen
NS1 is a non-structural protein of the dengue virus that is released into blood in high amounts during the first week of infection. NS1 antigen testing detects active, current dengue infection — usually positive from day 1 to day 7 of fever, with highest sensitivity in the first 3–4 days. It is the best single test for early dengue diagnosis in India, where dengue is endemic and peak during and after monsoon.
Expected Result
Normal
Negative
Most sensitive in the first 3–5 days of fever. After day 7, dengue IgM is the preferred test.
This is a qualitative test — results are reported as positive or negative rather than as a numeric range. Interpretation may vary by laboratory method; always review with your doctor.
What a Negative Result Means
A negative NS1 in the first 5 days of fever makes current dengue less likely but does not rule it out — sensitivity is around 70–90%. If dengue is clinically suspected despite a negative NS1, repeat NS1 or add dengue IgM and IgG from day 4 onwards. In secondary dengue infections, NS1 sensitivity is lower.
What a Positive Result Means
A positive NS1 confirms current dengue virus infection. Clinical priorities shift to: monitoring for warning signs (severe abdominal pain, persistent vomiting, bleeding, lethargy, hepatomegaly, rising hematocrit with falling platelets), hydration, avoiding aspirin and NSAIDs, and watching platelet counts. Most dengue is self-limiting over 7–10 days; severe dengue requires hospital admission.
Frequently Asked Questions
How early does NS1 turn positive?
NS1 becomes detectable from day 1 of fever and is highly sensitive through day 4–5. Sensitivity drops from day 6 onwards. If fever has been present for more than a week and dengue is suspected, NS1 is often negative — dengue IgM and IgG are better choices at that stage.
What is the difference between NS1 and dengue IgM?
NS1 is a virus protein — its presence means the virus is actively replicating. IgM is the body's antibody response, which appears from day 4–5 and peaks around day 10. Testing both NS1 and IgM together catches most cases across the illness timeline. IgM alone has a higher false-positive rate from cross-reactivity with other flaviviruses.
Can NS1 be positive without symptoms?
Very rarely. NS1 is usually tested only when fever and dengue-like symptoms prompt it. However, in large household cluster studies, some asymptomatic or minimally symptomatic infections have been detected. Routine NS1 screening without symptoms is not useful.
Related Infectious tests
See all →ASO Titre (Anti-Streptolysin O)
Antibody against streptococcal toxin — evidence of recent strep infection.
IU/mLInfectiousWidal Test
Antibody test for typhoid fever — used cautiously due to specificity issues.
InfectiousDengue IgM Antibody
Antibody that rises in the second week of dengue infection.
InfectiousDengue IgG Antibody
Antibody marking past or long-term dengue infection.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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