Labs & Reports · Fever & infection

Dengue Test: NS1, IgM and IgG, and Which One to Do on Which Day of Fever

A dengue test is a blood test that looks for the virus itself in the first days of fever, and for your antibodies after that. Which one you should do depends on a single fact: how many days you have had fever. Days one to five, the NS1 antigen test. From day five, the IgM antibody test. An IgG on its own mostly tells you about the past. If you have a report in your hand, the reading guide is a few screens down. If you are deciding whether to get tested today, start with the day count.

What is a dengue test, and what is it called on the lab form? Copy link

There is no single "dengue test". There are three blood tests that each catch a different stage of the infection, plus a molecular test that most people never need, and a blood count that travels with all of them. On a lab form or a receipt you will see one of these names, and the name tells you what the test is looking for.¹

If a form or an insurer asks for the dengue diagnosis test name, use the row that fits your day of fever, and add the CBC as a separate line. So when someone says "get a dengue test done," the useful reply is a question, as you would expect: which day of fever is it? That decides the name you should be asking the lab for. The next section is that decision.

Which dengue test should you do on which day? Copy link

Count from the first day of fever as day one. On days one to five, ask for the NS1 antigen test. From day five onward, ask for the IgM antibody test. Those are the windows India's national dengue guideline gives laboratories, and it asks the clinic to write the day of fever on the sample request so the lab runs the right one.¹ If you are on day four or five and the clinic is unsure, both together is reasonable, because that is exactly when one signal hands over to the other.

That is the Indian national program's testing algorithm. In clinical practice, especially around days four to seven or when a previous dengue infection is likely, a doctor may order NS1 and IgM together, because their windows overlap and combination testing catches more cases; the 2025 WHO guidance recommends exactly that.²

The question is not academic. India logged 2,33,519 confirmed dengue cases in 2024 and 1,21,824 in 2025, most of them between July and November. A large share of the testing behind those numbers happened in the first days of fever, when the choice of test decides whether the infection is seen at all.⁶ (West Bengal did not report for 2025, so that year's total understates the country.)

Why does the day matter so much? Because the virus and your immune system take turns. In the first days the virus is multiplying and shedding NS1 protein into your blood, and there is plenty of it to detect. Your antibodies have barely started. By day five or six the picture flips: antibodies are climbing and the virus is being cleared, so NS1 drops toward nothing. A test that looks for the wrong thing on the wrong day comes back negative even though you have dengue, and that is the single most common way these tests mislead people.²

Two refinements, both from the 2025 WHO laboratory guidance. NS1 can stay detectable up to about nine days in a first infection, but only to about six days if you have had dengue before, because the antibodies you already carry mop it up faster.² And IgM is not a switch that flips on day five. It is detectable in about half of patients by days three to five, and in most by the end of the first week, so an IgM done on day three is a coin toss.²

What does the NS1 antigen test show? Copy link

A positive dengue NS1 antigen test is strong evidence of a current acute dengue infection. NS1 is a protein the dengue virus makes and releases as it multiplies; it is not part of the virus particle itself, which is why the test can pick it up in large amounts early, before any antibody has formed.¹ A positive NS1 by ELISA counts as laboratory-confirmed dengue under India's case definition, on a single sample, with no second test needed.¹

How reliable is it? In a 2025 meta-analysis of 161 studies, the pooled sensitivity of NS1 ELISA in the first four days of symptoms was 90 percent, with a specificity of 93 percent, close to RT-PCR's 95 percent in the same window.³ Read that the way a clinician does: a positive is trustworthy, and a negative in the first four days still misses about one in ten people. The miss rate is higher in repeat infections, later in the week, and on rapid cards, all of which come up below.^2,3

People sometimes ask about the "NS1 antigen range" or a "normal value". There is none. The short answer is that the result is qualitative: positive, negative, or occasionally equivocal, in which case the lab will suggest a repeat. Some ELISA reports print an index number beside the word; a higher index does not mean worse dengue, and you should not try to track it. Any index above the kit's cut-off is simply positive.

What do dengue IgM and IgG mean? Copy link

IgM is your body's first antibody against the virus and it means recent infection. IgG is the long-lived antibody and it means exposure at some point, which could be this month or ten years ago. That difference in timescale is the whole art of reading the two together.

IgM. It usually becomes detectable by day five, though the guideline is candid that timing varies: some people show IgM on days two to four, others not until day seven or eight.¹ It peaks about two weeks in and then fades over two to three months; in some first infections it can still be found past 90 days.^1,2 India's 2023 national case definition classifies a clinically compatible case with a positive IgM ELISA on a single serum sample as confirmed dengue, and it is the test the national surveillance network runs, using kits supplied through NIV Pune.¹ International guidance is somewhat more cautious with an isolated IgM result, because dengue antibodies can cross-react with other flaviviruses, so the illness day, the local epidemiology and, in selected cases, paired samples or another confirmatory method still matter.^2,4 Its weakness, naturally, is the first week: pooled sensitivity across days one to seven is only 71 percent, which is why it should never be the only test ordered early.³

IgG. In a first infection, IgG appears late, around day ten to fourteen, at low levels, and then stays for life.² In a repeat infection with a different dengue serotype, the memory cells from the first time respond fast, so IgG rises within days and to high levels, often before or alongside IgM.² That is the practical reason labs report it at all during a fever. A high IgG in the first week is a marker of a second infection, and second infections carry a higher risk of the severe form. Your doctor may watch you more closely because of it.¹ The guideline itself calls IgG-ELISA a marker of past infection and not a diagnostic test on its own.¹ Some reports print an IgM to IgG ratio for the same purpose; treat it as a hint the doctor reads alongside the day of illness, not a verdict.

How do you read a dengue test report? Copy link

Read the three lines together with the day of fever, and the report almost always sorts itself into one of six patterns. Figure 2 draws them the way a rapid card shows them; the table beneath gives the same reading for an ELISA report, where the words will be positive, reactive or negative.

Notice what the table does not contain: a platelet count. The dengue tests tell you whether you have dengue. They say nothing about how you are doing, and a positive NS1 with a normal count on day two is the ordinary starting point of an illness that is usually mild. What decides how closely you are watched is the day of illness, the warning signs, and the trend in your blood count, which our page on the platelet count in dengue and beyond covers properly.

Is a positive dengue IgG without symptoms a problem? Copy link

Usually not. A lone positive IgG, with NS1 and IgM negative and no fever, is the signature of an infection you have already had and recovered from, quite possibly without ever knowing. In India that describes an enormous number of people. The national dengue serosurvey of 2017 to 2018 tested 12,300 people aged five to 45 across 15 states. It found dengue IgG in 48.7 percent of them, rising to 56.2 percent among adults aged 18 to 45 and 76.9 percent in the southern states.⁵ Most of those people had never been diagnosed with dengue. Their immune systems had simply met the virus and dealt with it.

This is why an IgG that turns up in a full-body package, or in a dengue profile ordered for a fever that turns out to be something else, so often causes needless alarm. In southern India, previous dengue exposure is extremely common: 76.9 percent of participants from the southern region in the national serosurvey carried IgG.⁵ Keep that in mind when it turns up on a report. It does not need treatment, it does not need repeating, and it does not mean you are protected: immunity after one serotype is lasting only against that serotype, and there are four.¹ What it does mean is that if this IgG truly reflects previous dengue and you later acquire dengue again, that would be a secondary infection, the pattern in which IgG rises fast and the risk of the severe form is higher. That is worth telling any doctor who treats you for a fever in future.

One honest limitation of the survey: indirect IgG ELISA cannot tell dengue antibodies from those raised by close relatives of the virus, and the authors say so.⁵ It is the same limitation that affects your own report, and it is the subject of the false-positive section below.

Rapid card test or ELISA: which one confirms dengue? Copy link

ELISA confirms. The card screens. That is not a matter of opinion; it is written into the case definition India uses. A positive NS1 or IgM on an ELISA counts as confirmed dengue, while a positive on a rapid card counts as probable dengue "due to poor sensitivity and specificity of currently available RDTs".¹ The guideline goes further and says the national program does not recommend rapid tests for diagnosing or managing dengue at all. Its reasons: most kits sold in India have never been properly validated, some independently evaluated kits gave a high rate of false positives, and accuracy varied from batch to batch.¹

In practice you will meet the card constantly, and it has real uses. It gives a result in 15 to 25 minutes at the bedside, and the better combined cards do well. A systematic review cited in the WHO guidance put the pooled sensitivity of NS1 plus IgM plus IgG rapid cards at 91 percent and specificity at 96 percent against reference tests.² The problem is that you cannot tell from the packet which kind of card your clinic bought. So the working rule is simple, and it is the one I use in clinic. A positive card means act as if it is dengue and ask for the ELISA. A negative card in the first two days rules out nothing. If money is tight and you can afford only one laboratory test, choose the day-appropriate ELISA, NS1 early in the illness and IgM later, or go to a sentinel hospital and ask what it offers free.

Can a dengue test be negative and you still have dengue? Copy link

Yes, and it happens for four ordinary reasons, none of which mean the lab made a mistake.

The test was done on the wrong day. This is the big one. An IgM on day two, or an NS1 on day nine, is looking for something that is not there yet or has already gone.^1,2 The fix is to repeat with the right test for the day, usually NS1 plus IgM around day five.

It is a repeat infection. If you have had dengue before, your existing antibodies bind the NS1 protein and clear it faster, so NS1 tests are less sensitive in second infections and the window closes around day six instead of day nine.² Given that half of Indian adults carry dengue IgG, this is a common way to get a false reassurance.

It was a rapid card. Even good cards miss some early cases, and a poor one misses many. A negative card with a strong clinical picture is a reason for an ELISA, not for relief.¹

The numbers are simply not perfect. Even NS1 ELISA in its best window misses about one in ten, and IgM ELISA in the first week misses about three in ten.³ Doctors know this. It is why the guideline tells clinics to manage a fever that looks like dengue as dengue while the results come in, and why anyone with fever for more than three days gets a blood count whatever the dengue test said.¹

Can a dengue test be wrong the other way? Copy link

Less often, but yes. NS1 ELISA and RT-PCR are highly specific: the US CDC notes that current molecular tests and NS1 ELISA do not cross-react with the other flaviviruses of concern, so a positive on either is close to definitive.⁴ Antibody tests are where the trouble lives. Dengue belongs to a family of viruses that includes Zika, Japanese encephalitis and yellow fever, and antibodies raised against one can react in a test for another. India's own guideline flags the IgM ELISA cross-reaction between dengue and Zika directly, and the CDC asks clinicians to check the patient's vaccination history for Japanese encephalitis and yellow fever before interpreting an antibody result.^1,4 If you had a JE vaccine as a child, as many people in endemic districts did, a lone IgG positive is even less informative than usual.

The other route to a wrong positive is the rapid card itself. The national guideline's reason for not recommending cards is precisely that some independently evaluated kits showed high false-positive rates against standard tests.¹ A positive card with a picture that does not fit dengue, say a fever with a runny nose and no body ache, deserves an ELISA before anyone writes dengue in your file.

What is the dengue test price in India? Copy link

Between nothing and about ₹1,000 for a single test, depending on where you go and which test it is. Through the government sentinel network the dengue fever test price is zero for the program-supported IgM ELISA, and often for NS1 where the site runs it. In private labs the charges for a dengue test vary widely by city and by method, and the method behind the name on the bill matters more than the name. For comparison, the current CGHS reimbursement schedule, in force since 13 October 2025, prices a dengue NS1 test at ₹400 and the IgM and IgG antibody pair at ₹800 in a Tier-I city such as Hyderabad, Delhi, Mumbai, Bengaluru or Chennai at a NABH-accredited provider. Its rates run 10 to 20 percent lower in smaller cities and 15 percent lower at non-accredited labs.⁷ These are government scheme rates rather than retail price caps, but they give you one useful reference point when comparing quotations.

Looking for a dengue test near you? The NCVBDC sentinel list is the free option, sorted by state, and many NABL-accredited laboratories offer or refer dengue ELISA; confirm that the method is ELISA rather than a rapid card. For a dengue fever blood test the price a national chain lists runs from ₹250 in Bengaluru to ₹1,050 in Chennai on one chain's site, and ₹600 is a common NS1 ELISA list price. In Hyderabad one large chain lists the NS1 ELISA at about ₹760 and the three-test ELISA profile at ₹1,969.^9,10,11,14 Private prices can sit above or below the CGHS schedule because that schedule is not a retail tariff. Ask what method is being used, ELISA or rapid card, and compare like with like. Booking online through a chain's app is convenient. The dengue, malaria and typhoid bundles they push in season, sometimes called a fever profile, cost more than the single test you need on most days, and a "serology" quote alone usually means the IgM and IgG pair without NS1. Check what is inside before you pay.

A few things the price lists do not say. The "dengue profile" or "dengue panel" most chains sell is NS1 plus IgM plus IgG on one sample. It is most useful around days four to seven, when more than one signal is plausible and WHO's algorithm allows NS1 and IgM together through the acute week.² On day one or two, the antibody half of that bundle is very likely to be negative, and you have paid for it anyway. Home collection usually adds a fee. And a card test priced like an ELISA is not a bargain; ask which method the lab uses, because the receipt often just says "Dengue NS1". The listed prices above were read from the labs' own pages in September 2026 and change with the season, so treat them as the shape of the market, and compare them with your own quotation before you pay.

What else does the doctor order with a dengue test? Copy link

A complete blood count, almost always, and for a reason that has nothing to do with confirming the diagnosis. The dengue tests answer "is it dengue?" once. The CBC answers "how is it going?" again and again, and that second question is the one that decides whether you rest at home or come in. The national guideline asks for a CBC with hematocrit and platelet count in everyone with warning signs and everyone with fever for more than three days.¹

Three values on that CBC report carry the weight. The platelet count, which falls through the febrile phase and bottoms out as the fever settles. The hematocrit, the packed-cell fraction of your blood, which rises if plasma starts leaking out of the vessels. A rising hematocrit matters more than a single absolute value: the national guideline treats roughly a 10 percent rise above baseline as an early indicator of plasma leakage, while a 20 percent rise appears in its management algorithm for dengue with warning signs. It is read alongside the hemoglobin.¹ And the white cell count, which typically drops early in dengue, with neutrophils low; its recovery is the first sign the illness is turning, a day or two before the platelets follow.¹ If you are admitted or have a related condition, liver enzymes, kidney function, electrolytes and blood sugar are added.¹

What should make you call the doctor between blood tests? The guideline's warning signs, and they are worth knowing by heart: persistent vomiting, abdominal pain or tenderness, bleeding from the nose or gums or in vomit or stool, unusual drowsiness or restlessness, fainting, cold clammy skin, difficulty breathing.¹ They tend to appear around the time the fever settles, commonly from about day three or four onward and through the following 24 to 48 hours, which is, of course, the moment many people relax. Do not. Check your temperature chart, and take the day the fever fell as the start of the period to watch most closely.

What if the result is negative and you still have fever? Copy link

Then the test has done its job and the search moves on. Fever in the monsoon months has a short list of usual suspects, and the guideline names them: malaria, typhoid, chikungunya, leptospirosis, scrub typhus, influenza and COVID-19, with more than one sometimes running at once.¹ Two of them deserve a specific word.

Malaria should be excluded early in any dengue-like fever because it has its own treatment and shares the low platelets and low white cells; a smear or rapid malaria antigen test is usually sent with the first sample. That is why labs bundle a blood test for dengue and malaria together, often with typhoid serology as a fever profile.¹ Typhoid is the other, and here timing bites again. The guideline points out that the Widal test may not turn positive before the second week of fever. So a negative Widal in week one does not clear typhoid, any more than an early negative NS1 clears dengue.¹ If that is where your fever is heading, our Widal test page explains what the titres mean and what to ask for instead.

And if the fever has already settled and you are reading this weeks later because your hair is coming out in the comb, that is a recognized after-effect of dengue, listed in the guideline's own table of manifestations. It is usually the delayed shedding called telogen effluvium, and it grows back.¹

Do you need to fast, and how long does the report take? Copy link

No fasting, no preparation. The dengue tests need a small tube of blood from a vein, drawn any time of day, and serum is the preferred sample.² Water, food and your usual medicines do not affect the result. A rapid card reads in 15 to 25 minutes; an ELISA is run in batches and most labs report it the same day or the next morning.¹ RT-PCR, where available, takes a day or two and is reserved for research, surveillance and a few hard cases.

Home collection is fine and widely offered. What matters is that the collector notes the day your fever started, so the lab runs the test that fits. If nobody asks you that question, tell them anyway. Note the same day count in your health check-up records too. A dengue IgG that appears on a routine panel next year will make far more sense with a date beside it.

Written by Dr. Tarun Reddy, MBBS · Registered medical practitioner (TSMC/FMR/36195) and clinical lead at Sapiens. Dr. Reddy writes the Labs & Reports journal with a standing rule that every threshold on the page is traceable to an Indian or primary source. Profile · Editorial standards