New Guidance Issued by CDC on Zika IgM Testing in Pregnancy

Janis C. Kelly

May 08, 2017

The Centers for Disease Control and Prevention (CDC) updated the 2016 Zika virus recommendations for clinicians caring for asymptomatic pregnant women with possible Zika exposure on May 5. The updated guidance incorporates new data showing that the immunoglobulin M (IgM) response to Zika can persist beyond 12 weeks, and is therefore not a reliable sign of recent infection. Conversely, Zika virus nucleic acid test (NAT) positivity fades over time as the Zika RNA level declines, so a negative NAT does not rule out recent infection.

"This change is being made because CDC's Zika testing guidance for pregnant women relies, in part, on a test [Zika virus Immunoglobulin M (IgM) ELISA] to detect Zika antibodies or proteins that the body makes to fight Zika infections," according to a CDC news release. "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 for some individuals. As a result, results of these tests may not be able to determine whether women were infected before or after they became pregnant."

This timing is important because the highest risk to the fetus is associated with infection during the first trimester of pregnancy.

The updated guidance changes the recommendations for testing asymptomatic pregnant women who might have been exposed to Zika before conception. Recommendations for testing pregnant women who are symptomatic for Zika remain unchanged.

"However," the CDC warns, "if a symptomatic pregnant woman is IgM positive and NAT negative, and lived in or traveled to an area with a posted CDC Zika Travel Notice, healthcare providers should recognize that the positive IgM result does not necessarily indicate recent infection."

Five Steps for Testing Asymptomatic Pregnant Women

The CDC recommends five steps for testing asymptomatic pregnant women with possible Zika exposure before conception, including women who have lived in or frequently (daily or weekly) travel to areas included in CDC travel warnings.

  1. Screen all pregnant women for Zika exposure risk and symptoms, with prompt Zika NAT for those who become symptomatic during pregnancy or whose sexual partner tests positive for Zika infection.

  2. Test Zika NAT at least every trimester unless a previous test was positive.

  3. If amniocentesis is performed for a reason other than Zika, consider also conducting Zika NAT of the amniocentesis specimens.

  4. Each trimester counsel pregnant women on the limitations of Zika IgM and NAT testing.

  5. Consider preconception IgM testing to determine baseline Zika virus IgM levels as part of preconception counseling.

Current Zika infection in a pregnant woman is associated with increased risk to the fetus, but the CDC advisory notes that prolonged IgM responses to Zika, as in other flavivirus infections, complicate efforts to distinguish recent from prior infections in areas with endemic transmission. Timing of infection may be further obscured by cross-reactivity with other flaviviruses, particularly dengue.

Data from studies of NAT-confirmed Zika in symptomatic patients in Puerto Rico, where dengue is not a factor, found a median time to first negative IgM of 122 days after symptom onset (range, 8 - 210 days). NAT testing in those patients showed viral RNA in 36% of patients 8 to 15 days after symptom onset, but three of five pregnant women in that study had detectable RNA at 46 days, and one at 80 days, after symptom onset.

"Our guidance today is part of our continued effort to share data for public health action as quickly as possible," Henry Walke, MD, incident manager of the CDC's Zika response efforts, said in the news release. "As we learn more about the limitations of antibody testing, we continue to update our guidance to ensure that healthcare professionals have the latest information for counseling patients who are infected with Zika during pregnancy."

CDC Health Alert Network (HAN) Health Advisory, no. 402. Published online May 5, 2017. Full text

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