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Testing & Diagnosis

Health Alert Notice: CDC issued a Health Alert Notice (HAN) to share emerging evidence about interpreting Zika IgM antibody test results of pregnant women who may have been exposed to Zika virus, particularly women who live in or frequently travel to areas with a CDC Zika travel notice. It is possible that some women who are currently pregnant may have been previously infected and developed antibodies against Zika prior to pregnancy. New data suggest that Zika virus infection, similar to some other flavivirus infections, may result in Zika antibodies staying in the body for months after infection, which may make it difficult to use these tests to determine whether women might have been infected before or after they became pregnant. This HAN has specific recommendations not currently a part of the existing laboratory guidance, which should be considered for these women:  1. that nucleic acid testing is considered at least once per trimester unless a previous test has been positive, and on amniocentesis specimens, if amniocentesis is performed for other reasons and 2. that IgM testing may be considered as part of pre-conception counseling. CDC recommends other diagnostic methods, such as nucleic acid testing and ultrasounds, which may provide additional information to help healthcare providers know if antibody test results might represent a recent infection. CDC is currently updating its webpages with this information.

Who to Test

All pregnant women in the United States and US territories should be assessed for possible Zika virus exposure at each prenatal care visit. Possible exposure to Zika virus that warrants testing includes

  • Travel to or residence in an area with risk of Zika, or
  • Sex (vaginal, anal, and oral sex) without a condom, or sharing sex toys with a person who traveled to or lives in an area with risk of Zika.

Laboratory evidence of a confirmed recent Zika virus infection includes

  1. Detection of Zika virus or Zika virus RNA or antigen in any body fluid or tissue specimen; or
  2. Positive or equivocal Zika virus or dengue virus IgM test on serum or cerebrospinal fluid (CSF) with a positive titer for Zika virus (≥10) from plaque reduction neutralization testing (PRNT) together with negative PRNT titer (i.e., <10) for dengue virus. Interpretation of serologic results has been described and published elsewhere.

When to Test

If your patient… When to be tested
Traveled to or had sex with a partner who has lived in or traveled to an area with risk of Zika virus infection that has a CDC Zika travel notice

Pregnant women who traveled to or had sex with a partner who has lived in or traveled to an area with a CDC Zika travel notice should be tested for Zika after their exposure, regardless of whether or not they have symptoms.

Traveled to or had sex with a partner who has lived in or traveled to an area with risk of Zika virus infection but no CDC Zika travel notice

Pregnant women who traveled to or had sex with a partner who has lived in or traveled to an area with risk of Zika should be tested if they develop symptoms of Zika or if their fetus has abnormalities on an ultrasound that may be related to Zika infection. Because the level of risk of Zika virus infection is unknown in areas with Zika risk but without travel notices, routine testing is not recommended for pregnant women who have exposure to those areas but who do not have symptoms. However, testing may be offered on a case-by-case basis.

Lives in or frequently travels to an area with risk of Zika virus infection that has a CDC Zika travel notice

Pregnant women who live in an area with a Zika travel notice are at risk of getting Zika throughout pregnancy. For this reason, testing should be offered

  • At the first prenatal care visit and
  • A second test in the second trimester.

If a pregnant woman has symptoms of Zika at any time during her pregnancy, she should be tested for Zika.

Testing by Geographic Location


Testing Symptomatic Pregnant Women

Pregnant women who report signs or symptoms consistent with Zika virus disease (acute onset of fever, rash, arthralgia, conjunctivitis) should be tested for Zika virus infection. The testing recommendations for symptomatic pregnant women are the same regardless of the circumstances of possible exposure; however, the type of testing recommended varies depending on the time of evaluation relative to symptom onset.

Symptomatic pregnant women who seek care <2 weeks after symptom onset should receive testing of serum and urine by RNA nucleic acid testing (NAT; e.g. rt-PCR). A positive RNA NAT result confirms the diagnosis of recent maternal Zika virus infection. Symptomatic pregnant women with negative RNA NAT results should receive both Zika virus IgM and dengue virus IgM antibody testing. If Zika virus RNA NAT testing is requested from laboratories that do not have IgM antibody testing capacity or a process to forward specimens to another testing laboratory, storing of additional serum samples is recommended for IgM antibody testing in the event of a negative RNA NAT result. If either the Zika virus or dengue virus IgM antibody test yields positive or equivocal results, PRNT should be performed on the same IgM-tested sample or a subsequently collected sample to rule out false-positive results.

Symptomatic pregnant women who seek care 2–12 weeks after symptom onset should first receive Zika virus and dengue virus IgM antibody testing. If the Zika virus IgM antibody testing yields positive or equivocal results, reflex RNA NAT testing should be automatically performed on the same serum sample to determine whether Zika virus RNA is present. A positive RNA NAT result confirms the diagnosis of recent maternal Zika virus infection. However, if the RNA NAT result is negative, a positive or equivocal Zika virus IgM antibody test result should be followed by PRNT. Positive or equivocal dengue IgM antibody test results with a negative Zika virus IgM antibody test result should also be confirmed by PRNT. Interpretation of serologic results has been described.

Testing Asymptomatic Pregnant Women

Testing recommendations for asymptomatic pregnant women with possible Zika virus exposure differ based on the circumstances of possible exposure (i.e., ongoing versus limited exposure) and the elapsed interval since the last possible Zika virus exposure.

Asymptomatic pregnant women living in areas without Zika who are evaluated <2 weeks after possible Zika virus exposure should be offered serum and urine RNA NAT testing. A positive RNA NAT result confirms the diagnosis of recent maternal Zika virus infection. However, because viral RNA in serum and urine declines over time and depends on multiple factors, asymptomatic pregnant women with a negative RNA NAT result require additional testing to exclude infection. These women should return 2–12 weeks after possible Zika virus exposure for Zika virus IgM antibody testing. A positive or equivocal IgM antibody test result should be confirmed by PRNT.

Asymptomatic pregnant women living in an area without Zika, who seek care 2–12 weeks after possible Zika virus exposure should be offered Zika virus IgM antibody testing. If the Zika virus IgM antibody test yields positive or equivocal results, reflex RNA NAT testing should be performed on the same sample. If the RNA NAT result is negative, PRNT should be performed.

NOTE: Testing for asymptomatic pregnant travelers with Zika virus exposure who have traveled to an area with risk of Zika but no CDC Zika travel notice is not routinely recommended. False positive results are more likely to occur when there are lower levels of Zika virus transmission or co-circulation of other flaviviruses. Positive Zika virus serology results require additional confirmatory testing and these results might take several weeks. Healthcare providers can offer testing on a case-by-case basis for asymptomatic pregnant women who are concerned about Zika virus infection. However, pretest counseling should clearly convey the limitations associated with interpretation of test results.

Asymptomatic pregnant women who have an ongoing risk for Zika virus exposure (i.e., residence in or frequent travel to an area with risk of Zika) should receive IgM antibody testing as part of routine obstetric care during the first and second trimesters. Reflex RNA NAT testing is recommended for women who have a positive or equivocal Zika virus IgM antibody test results because RNA NAT testing provides the potential for a definitive diagnosis of Zika virus infection. Negative RNA NAT results after a positive or equivocal Zika virus IgM antibody test result should be followed by PRNT. The decision to implement testing of asymptomatic pregnant women with ongoing risk for Zika virus exposure should be made by local health officials based on information about levels of Zika virus transmission and laboratory capacity.

Testing Partners of Pregnant Women

CDC recommends Zika virus testing for anyone who is not pregnant who has been exposed to Zika and who also has Zika symptoms.

Testing blood, semen, vaginal fluids, or urine is not recommended to determine how likely a person is to pass Zika virus through sex. Because Zika virus can remain in some fluids (for example, semen) longer than blood, someone might have a negative blood test, but still carry Zika in their genital secretions.

Intermittent shedding in semen can occur with other viruses and the pattern of Zika virus shedding in semen is unknown. In addition, the detection of Zika virus RNA in semen might not indicate the presence of infectious virus in semen. Testing semen and vaginal fluids for Zika virus is not currently available outside of the research setting. Studies are underway to better understand the performance of these tests, the persistence of Zika virus in these fluids, and how best to interpret the results.

Testing Information

For more information on diagnostic testing for Zika, see Testing for Zika.

Specimens

Zika virus RNA NAT and serology assays can be performed on maternal serum or cerebrospinal fluid. Zika virus RNA NAT can also be performed on maternal whole blood, amniotic fluid, plasma, and urine. Other testing that can be performed includes the following: 1) histopathologic examination and immunohistochemical staining of the placenta and umbilical cord, and 2) Zika virus testing of frozen placental tissue and cord tissue. For a number of reasons, the use of cord blood to diagnose Zika virus infection is not recommended.

Testing Challenges

RNA NAT testing may not demonstrate Zika virus RNA in a woman with Zika virus infection if the period of viremia has passed. Serum serologic testing can be performed, however, cross-reactivity with related flaviviruses (e.g., dengue and yellow fever viruses) is common. Plaque-reduction neutralization testing (PRNT) can be performed to measure virus-specific neutralizing antibodies to Zika virus, but neutralizing antibodies may still yield cross-reactive results in persons who were previously infected with another flavivirus, such as dengue, or has been vaccinated against yellow fever or Japanese encephalitis. In addition, new data suggest that Zika virus infection, similar to some other flavivirus infections, may result in Zika antibodies staying in the body for months after infection, which may make it difficult to use these tests to determine whether women might have been infected before or after they became pregnant. This new information is particularly relevant for pregnant women without symptoms who had potential exposure to areas with Zika virus, particularly women that lived in or traveled to a place with a CDC Zika travel notice, before they became pregnant. It is important to work closely with your state or local health department to ensure the appropriate test is ordered and interpreted correctly.

Ordering Tests

Each clinical scenario is unique, and healthcare providers should consider all available information when ordering a test for Zika virus infection, including patient travel history or possible exposure through sexual contact, history of flavivirus infection, vaccination history, ultrasound findings, and the presence of symptoms. Providers should work with their state, local, and territorial health departments for assistance obtaining and interpreting test results.

All laboratory testing requests and results reports for pregnant women should clearly indicate pregnancy status. This will facilitate prioritization of testing, ascertainment of pregnancies affected by Zika, and consistent interpretation of laboratory results.

Healthcare providers should work closely with the state, local, or territorial health department to ensure that the appropriate test is ordered and interpreted correctly. In addition, CDC maintains a 24/7 Zika consultation service for health officials and healthcare providers caring for pregnant women. To contact the service, call 770-488-7100 and ask for the Zika Pregnancy Hotline or email ZIKAMCH@cdc.gov.

Clinician Resources

Updated Interim Pregnancy Guidance

Zika Pregnancy Testing Algorithm


Zika Pregnancy Screening Tool

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