Dengue Fever Testing

Dengue fever testing is used to determine whether a person with signs and symptoms and recent potential exposure has been infected with the dengue virus. The infection is difficult to diagnose without laboratory tests because symptoms may initially resemble those of other diseases, such as chikungunya infection. Two primary types of testing are available:

• Molecular testing (polymerase chain reaction, PCR)—this type of test detects the genetic material of the dengue virus in blood within the first week after symptoms appear (fever) and can be used to determine which of the 4 serotypes is causing the infection. One type of Real Time RT-PCR test can detect dengue and the two other mosquito-borne viruses, Zika and chikungunya, and distinguish between the three. Only certain public health laboratories are able to provide the test after verifying that they can successfully perform the assay. Though the test is not available in hospitals or clinics, healthcare practitioners are able to order it through their state and local public health departments. Results can take from four days to two weeks, according to the Centers for Disease Control and Prevention (CDC).

  Molecular tests of blood are not likely to detect the virus after 7 days of illness. If the result of a PCR test is negative, an antibody test can be used to help establish a diagnosis, according to the CDC (see below).

• Antibody tests—these tests are primarily used to help diagnose a current or recent infection. They detect two different classes of antibodies produced by the body in response to a dengue fever infection, IgG and IgM. Diagnosis may require a combination of these tests because the body's immune system produces varying levels of antibodies over the course of the illness.

  IgM antibodies are produced first and tests for these are most effective when performed at least 7-10 days after exposure. Levels in the blood rise for a few weeks, then gradually decrease. After a few months, IgM antibodies fall below detectable levels.

  IgG antibodies are produced more slowly in response to an infection. Typically, the level rises with an acute infection, stabilizes, and then persists long-term. Individuals who have been exposed to the virus prior to the current infection maintain a level of IgG antibodies in the blood that can affect the interpretation of diagnostic results.

Testing may be ordered when individuals have signs and symptoms associated with dengue following travel to tropical locations where the dengue virus is present. Some of the main signs and symptoms include:

• Sudden high fever (104°F or 40°C)
• Severe headache or pain behind the eyes
• Joint, muscle and/or bone pain
• Gum and nose bleeds
• Easy bruising
• Low white blood cell count
   

Molecular testing is ordered within one week of the onset of symptoms to detect an acute infection, while antibody testing may be ordered more than 4 days after symptoms appear. If antibody testing is performed, an additional blood sample may be collected after two weeks of symptoms to determine if the antibody level is rising.

Molecular testing—a PCR test that detects the presence of the virus itself is generally considered the most reliable means of diagnosis. A positive result from a PCR is considered conclusive. A negative result on a PCR test may indicate that no infection is present or that the level of virus is too low to detect, as may happen if the test was performed after the 7-day window during which the virus is present in the sample collected for this test. A negative PCR result is followed by antibody testing (below).

Antibody testing—antibody tests may be reported as positive or negative, or may be reported as an antibody titer with an interpretation of which type(s) of antibody (IgG or IgM) is present.

Positive IgM and IgG tests for dengue antibodies detected in an initial blood sample mean that it is likely that the person became infected with dengue virus within recent weeks. IgM antibody tests can be positive if a person has been infected with a similar virus, such as chikungunya (called cross-reaction). If an initial IgM antibody test is positive, a second test called the Plaque Reduction Neutralization Test (PRNT) is used to confirm the presence of antibodies to dengue virus and to help rule out other viral infections.

If the IgG is positive but the IgM is low or negative, then it is likely that the person had an infection sometime in the past. If the dengue IgG antibody titer increases four-fold or greater (e.g., titer of 1:4 to a titer of 1:64) between an initial sample and one taken 2 to 4 weeks later, then it is likely that a person has had a recent infection.

Negative tests for IgM and/or IgG antibodies may mean that the individual tested does not have a dengue infection and symptoms are due to another cause, or that the level of antibody may be too low to measure. The person may still have a dengue infection – it may just be that it is too soon after initial exposure to the virus to produce a detectable level of antibody.

The following table summarizes results that may be seen with antibody testing:

Physical symptoms like rash or aching joints are not a reliable means for diagnosing dengue fever because the symptoms are not likely to appear until after the initial fever has passed.

Antibody tests for dengue fever can be positive if a person is infected with another arbovirus such as West Nile virus. A healthcare practitioner will consider a person's test results, medical history, and recent travel history in making a diagnosis.

No laboratory test can predict whether or not the infection will progress to the more severe form, but those who have been previously infected with dengue are at an increased risk for developing severe dengue during the second infection.

Currently, there is no immunization that will prevent a person from contracting dengue fever if exposed to the virus. Limiting exposure to the virus depends upon protecting against mosquito bites. When traveling in tropical climates, wear insect repellent that contains DEET and long sleeved-shirts and long pants. Stay indoors during dawn and dusk when mosquitoes are most active.