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New Zika Testing Strategy to Avoid False Positives in Pregnant Women

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, August 4, 2017

In the wake of waning case counts of Zika in the Americas and the discovery that IgM antibodies persist for periods longer than 12 weeks, the Centers for Disease Control and Prevention (CDC) has released new interim guidance for testing pregnant women.


False-Positive Risk, IgM Persistence

As the prevalence of any condition diminishes in a given geographic area, the likelihood that a positive diagnostic test result is a true positive—as opposed to a false positive—decreases. When a disease is rare but testing is quite frequent, false positives may far exceed true positives. This is the current situation with Zika on the US mainland. Because a false-positive test result could have profound implications for a pregnant woman and her fetus, it is essential to optimize testing and counseling.

Coupled with the increasing chance of false-positive test results is the fact that IgM serological testing can remain positive for over 12 weeks. An isolated positive IgM result could, therefore, represent infection prior to pregnancy and would not be reflective of any risk to the developing fetus.

In this context, new CDC recommendations include:

  • Inquiring about Zika exposure at every prenatal visit before and during pregnancy
  • Testing pregnant women with possible Zika symptoms and exposure with combined nucleic acid (blood and urine) and IgM serological testing as soon as feasible up to 12 weeks after symptom onset
  • Testing asymptomatic pregnant women with ongoing Zika exposure 3 times during pregnancy, with the first test being at initiation of prenatal care
  • Not testing asymptomatic pregnant women without ongoing Zika exposure
  • Testing women with fetal ultrasound findings consistent with congenital Zika syndrome to determine etiology
  • Not using IgM screening of nonpregnant women as part of preconception counseling


Guidelines Reflect Dynamism of Emerging Infectious Diseases

The updated guidelines reflect the dynamic nature of an emerging infectious disease outbreak in which new knowledge is constantly acquired and epidemiologic conditions change. It can and should be anticipated that guidelines for these types of outbreaks will never be static, and constant situational awareness on the part of guideline makers and clinicians is required to ensure the best application of guidelines and their continual improvement.



Oduyebo T, Polen KD, Walke HT, et al. Update: Interim guidance for health care providers caring for pregnant women with possible Zika virus exposure—United States (including U.S. territories), July 2017. MMWR Morb Mortal Wkly Rep 2017;66(29):781-793. Accessed August 2, 2017.