Dengue Test: NS1, IgM and When to Get Tested

Dengue in brief

Dengue is a viral infection spread by the bite of infected Aedes mosquitoes, which bite mainly during the day. WHO notes that most infected people have mild or no symptoms and recover in one to two weeks. When symptoms occur, they usually begin 4 to 10 days after the bite and last 2 to 7 days.

Common symptoms include high fever, severe headache, pain behind the eyes, muscle and joint pain, nausea, vomiting, swollen glands and a rash. A second dengue infection carries a higher risk of severe disease. Because early dengue looks like many other fevers, including malaria, typhoid and chikungunya, the Indian guidelines stress testing to tell them apart.

Dengue test names: NS1, IgM, IgG and PCR

• NS1 antigen test. NS1 is a protein made by the dengue virus that is abundant in the blood in the early stage of infection. The national guidelines describe the ELISA-based NS1 test as simple, highly specific and highly sensitive, allowing early detection.
• IgM antibody test (MAC-ELISA). IgM antibodies are made by the body in response to the virus. They are usually detectable by day 5 of illness, but this varies: some people have them by day 2 to 4, others not until day 7 or 8. IgM can stay in the blood for weeks, usually fading by about 60 days.
• IgG antibody test. IgG indicates a past infection and helps tell a first infection from a repeat one. The guidelines do not treat it as a diagnostic test on its own.
• PCR (molecular test). PCR detects the virus's genetic material. MedlinePlus notes it works during the first seven days after symptoms begin.

When to do the dengue test after fever

The national guidelines tell laboratories to choose the test by the day of illness:

That is why the lab will ask the date your fever started. Count the first day of fever as day 1 and tell the staff clearly. An IgM test done too early can be negative even when dengue is present.

MedlinePlus adds that a negative antibody test done too early may need to be repeated after a few days if dengue is still suspected. No fasting or special preparation is needed for dengue tests.

Rapid card tests versus ELISA

Rapid diagnostic test (RDT) kits for NS1 and IgM/IgG give a result in 15 to 25 minutes. However, the national guidelines report that their accuracy varies, including between batches, and that some show many false positives. For this reason the national programme does not recommend rapid tests for diagnosing or guiding treatment of dengue. A positive rapid test is classed only as "probable" dengue.

A case is confirmed by an ELISA-based NS1 or IgM test, by PCR, or by virus isolation. When you get a report, check which method was used, and ask your doctor whether a confirmatory test is needed.

CBC and platelet count in dengue

Alongside the dengue test, doctors order a CBC (complete blood count), often repeatedly. The national guidelines advise follow-up with CBC and haematocrit to track the illness. In dengue the CBC can show a fall in white cells and platelets. The pattern that most concerns doctors is a rising haematocrit together with a rapidly falling platelet count, which the guidelines list as a warning sign of plasma leakage.

Many families focus on the platelet number alone. The guidelines caution that platelet counts are unreliable for predicting bleeding, and that preventive platelet transfusion was not better than supportive care in a trial of adults without significant bleeding. Decisions about admission and transfusion should be left to the treating doctor, who looks at the whole picture.

Warning signs: go to hospital immediately

The critical phase of dengue often begins around day 3 to 4 of fever, and sometimes just as the fever comes down, so a falling temperature does not always mean recovery. The national guidelines ask families to watch for:

• Severe abdominal pain or persistent vomiting
• Red spots or patches on the skin
• Bleeding from the nose or gums, vomiting blood, or black tarry stools
• Drowsiness, irritability or restlessness
• Pale, cold or clammy skin
• Difficulty in breathing

Any of these needs urgent hospital care. MedlinePlus also lists pregnancy, age over 65 and a previous dengue infection as risk factors for severe dengue, so these patients should see a doctor early.

Care at home for mild dengue

• Rest, and drink enough fluids such as water, ORS or coconut water; the guidelines advise avoiding carbonated drinks.
• Use paracetamol for fever and pain only in the dose your doctor advises. WHO and the national guidelines say to avoid aspirin, ibuprofen and other NSAIDs, which can increase bleeding.
• Keep follow-up visits and repeat blood tests as advised, and watch for the warning signs above.

To prevent dengue, WHO advises protecting against daytime mosquito bites with covering clothes, repellents, nets and window screens, and emptying, cleaning and covering water containers every week so mosquitoes cannot breed.

Dengue testing at MCT Clinic and Diagnostic Centre

A dengue test is listed in our tests catalogue under Dengue Test, along with malaria, typhoid and fever panels and the CBC. When you come, tell the staff the date your fever began so the right test can be chosen, and ask which method is used for your sample. Our physician and paediatrician can review the result. For a child, see our paediatric consultant rather than waiting for the report at home.