What it is
Dengue tests are blood tests. Some look for the virus itself: an NS1 antigen test finds a protein of the virus, and some labs use an RT-PCR test that finds its genetic material. Others look for the body's response: IgM antibodies appear first and IgG later. A blood count with the platelet count does not diagnose dengue. It shows how the illness is affecting the blood.
Why it is done
Early dengue looks like many other fevers, including chikungunya, malaria, typhoid and leptospirosis, so tests help to tell them apart. Doctors advise a blood count with and haematocrit (the share of the blood that is red cells) for anyone with a warning sign or fever for more than 3 days, and repeat it as the illness goes on.
What it helps with
How to prepare
A blood count needs no special preparation. Ask the lab about the other tests. Note the day your fever began and tell the lab, as it uses this to choose the test.
Do
- Tell the doctor about every medicine you take, especially any that thin the blood
- Drink plenty of fluids, as your doctor advises
Do not
- Do not take aspirin or ibuprofen for fever, as they can raise the risk of bleeding
- Do not stop a regular medicine unless the doctor tells you to
What happens
- You tell the nurse the day your fever began.
- Blood is drawn from a vein in your arm with a small needle. You may feel a brief sting. It usually takes under five minutes.
- The blood goes to the lab, which runs the test chosen for the day of your fever.
| Day of fever | Test | Note |
|---|---|---|
| Days 1 to 5 | NS1 antigen | Finds a viral protein while the virus is in the blood |
| After day 5 | IgM antibody | Usually found by day 5, sooner in some people, only after 7 to 8 days in others |
| Any day | Blood count | Platelets and haematocrit, repeated as needed |
Quick card tests give an answer in 15 to 25 minutes, but the national programme does not recommend them to diagnose dengue or guide treatment, as many have not been properly checked and some give false positives.
Afterwards
You can usually go home straight away. While you wait for results, rest, drink plenty of fluids and use paracetamol for fever if the doctor advises it. Watch for the warning signs above: severe illness often shows up after the fever has gone down, so a repeat blood count may be asked for even when you feel better.
Risks and side effects
Blood tests carry very little risk: slight pain or bruising where the needle went in, which fades quickly. The bigger risk is a misleading reading. A test taken on the wrong day can be negative in someone who has dengue.
Results and what comes next
The doctor reads the result together with how ill you are, the days of fever and your blood count. A negative result early on does not rule dengue out, so the test may be repeated, or the doctor may go by the illness itself. A positive IgM shows dengue infection in the last two or three months. IgG alone shows only a past infection.
A falling platelet count with a rising haematocrit can show that the illness is getting worse. Platelet counts do not predict bleeding well. The report prints the lab's own normal range, which depends on age, sex and the method.
Questions to ask your doctor
Tick the ones you want answered and take them with you.
Common questions
Which dengue test should I have on the second day of fever?
The national guideline says an NS1 antigen test suits samples taken on days 1 to 5 of fever, and an IgM antibody test suits samples taken after day 5. The lab or doctor will choose, so tell them the day your fever began.
Does a low platelet count mean I have dengue?
No. Low platelets can have other causes, including some other viral infections. Doctors read the platelet count together with your fever, your symptoms and a test for the dengue virus or antibodies.
Do I need platelets if my count is low?
Not by itself. Platelet counts do not predict bleeding well, and the national guideline advises giving platelets only when there is bleeding, with an exception the doctor may consider for a very low count. The doctor decides, so ask what your count means.
Can I rely on a quick card test for dengue?
The national programme does not recommend quick tests to diagnose dengue or guide treatment, because their accuracy is not well established and some give false positives. A positive quick test is treated as probable dengue, so ask your doctor whether a laboratory test is needed.
