The U.S. Centers for Disease Control and Prevention (CDC) on May 5 issued a health advisory with updated recommendations for Zika virus testing in pregnant women based on emerging evidence about the virus causing sustained viral response in some people. New data show that like other flaviviruses, Zika virus immunoglobulin M (IgM) sometimes persists longer than 12 weeks in certain patients, which complicates IgM test interpretation for all infected people. 

A recent study of symptomatic patients in Puerto Rico with nucleic acid test (NAT)-confirmed Zika infection found that IgM was still detectable in 87% of them 60 days after symptom onset. Unpublished data show a median time of 4 months to first negative IgM result.

Since Zika infection has the most profound consequences for pregnant women, CDC’s advisory focuses on testing this patient population with five key recommendations not already part of its lab guidance:    

  • Screen pregnant women for risk of Zika and test them with a NAT if they become symptomatic or if their sexual partners test positive for Zika virus infection.
  • Consider NAT testing during each trimester if prior tests have been negative.
  • Consider NAT testing of amniocentesis specimens if this procedure is already being performed.
  • Counsel women during each trimester about the limitations of both IgM and NAT testing.
  • Consider determining baseline viral IgM levels as part of preconception counseling.

A negative NAT test does not rule out recent infection because viral RNA declines over time, according to CDC. The agency emphasized as well that cross-reactivity with other flaviviruses, especially dengue virus, complicates IgM test results when an individual has been infected or vaccinated for these Zika-related viruses.

The health advisory is available online at