Zika Virus Testing

Zika virus testing is used to detect a Zika infection in a person without signs and symptoms and to determine whether a person with signs and symptoms of Zika has been infected after exposure in a region with Zika virus. It may also be used to test people who have had sexual contact with a recent traveler to a country with Zika. 

Zika infection may be difficult to diagnose without laboratory tests because symptoms may resemble those of other diseases, such as dengue fever or chikungunya infection, or the patient may be asymptomatic. Currently, two types of testing are available:

• Molecular tests for Zika virus (PCR)—these detect the presence of the RNA of the virus and can diagnose Zika infection if tested within a week after the onset of symptoms. One type of Real Time RT-PCR test can detect Zika and the two other mosquito-borne viruses, dengue and chikungunya, and distinguish between the three. This test can be used on blood, cerebrospinal fluid (CSF), amniotic fluid, or urine samples.

  Molecular tests of blood are not likely to detect Zika after 7 days of illness. Urine testing can detect Zika RNA for at least two weeks after symptoms begin. According to a recent study, urine may be the preferred specimen for detecting Zika RNA during the first three weeks of illness. If urine and blood specimens are taken less than 7 days after the onset of illness, the Centers for Disease Control and Prevention (CDC) recommends testing both samples.

  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 blood tests—these detect the IgM class of antibodies produced by the immune system when a person has been exposed to the virus. An antibody test may be used following a negative result on a PCR test or as alternative test for individuals who may have been infected but the length of time has passed when PCR can be performed. IgM antibodies appear in the blood 4 to 5 days after the start of the illness and last for about 12 weeks. Performing the test outside this window can yield false-negative results.

  IgM antibody tests can be positive if a person has been infected with a similar virus, such as dengue (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 Zika virus and to help rule out other viral infections.
   

The amount of time that it takes for test results to become available can vary depending on where the person being tested is located and whether the state public health laboratory is performing Zika testing. Some samples may need to be sent to the CDC for testing. In general, it may take about three weeks for results, according to the CDC.

Zika testing may be ordered when an individual has signs and symptoms associated with Zika infection and recently traveled to areas where the virus is actively spread by mosquitos or had sexual contact with someone who recently traveled from an area where Zika is present.

Signs and symptoms of Zika typically last between 2 and 7 days. Some of the main signs and symptoms include:

• Fever
• Joint pain
• Rash
• Conjunctivitis (pink eye)
• Headache
   

According to CDC guidelines, asymptomatic pregnant women who have recently traveled to an area with active Zika transmission may be offered the antibody test. Pregnant women who have had sexual contact with a recent traveler to a country with Zika may also be tested.

Testing may be performed when a newborn has microcephaly (an abnormally small head). If a person has Guillain-Barre syndrome and is at risk for Zika virus, that person may be tested.

Molecular testing

• A PCR test that detects the RNA of the virus is generally considered the most reliable means of diagnosis. A positive PCR result for Zika is considered conclusive. All positive results should be reported to the state public health department.
• A negative PCR result may indicate that no infection is present or that the level of virus is too low to detect. This can happen if the blood test is performed after the 7-day window and/or the urine test is performed after the 2 to 3-week window during which the virus is present in the sample. The CDC recommends that antibody testing be done following a negative PCR result (see below).
   

Antibody testing

• A positive IgM test for Zika antibodies is followed by a second test called the Plaque Reduction Neutralization Test (PRNT), which is used to confirm the presence of antibodies to Zika virus. No patient management decisions should be made based on IgM blood tests (serology) alone until the PRNT is completed because of the risk of false-positive results. A PRNT positive for Zika virus and negative for dengue virus is indicative of recent Zika virus infection.
• Negative tests for IgM antibodies do not rule out the possibility of Zika virus infection. A negative result may mean that the individual tested does not have a Zika infection and symptoms are due to another cause. It may also mean that the level of antibody may be too low to measure. The person may still have a Zika infection; it may just be that it is too soon after initial exposure to the virus to produce a detectable level of antibody. Early in the infectious process before the production of antibodies, PCR testing (above) may be performed when testing is done within 7 days of illness.
   

Since information about Zika is changing quickly, check the CDC's web site for the latest on the virus and testing.

Tests for the Zika virus or antibodies do not provide any information about fetal health in pregnant women with Zika virus.

Physical symptoms like rash or aching joints are not a reliable means for diagnosing Zika because the symptoms are not specific to Zika.

The U.S. Food and Drug Administration (FDA) has directed blood collection facilities to screen all donated blood and blood components for the Zika virus. The FDA approved two investigational tests for this purpose. The tests were approved under an "investigational new device (IND)" protocol and did not go through the standard regulatory process prior to implementation, which can be time-consuming. The tests screen for the presence of the virus by detecting its genetic material (RNA). Units of blood that test positive for the virus are removed from the supply and not used for transfusions.

The amount of time that it takes for results to become available can vary depending on where you are located and whether your local or state public health laboratory is performing Zika testing. Some samples may need to be sent to the Centers for Disease Control and Prevention (CDC) for testing. Test results may be available within 3 weeks. However, it may take longer during the summer when arbovirus (viruses spread by bites of mosquitos and ticks) activity is high.