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here’s never been anything easy about the Zika virus outbreak, and a new complication is now coming to light.

Testing for Zika infection is becoming more difficult, making it harder for doctors to advise pregnant women about the chances their child might have a Zika-related birth defect, the Centers for Disease Control and Prevention revealed in a health advisory issued Friday.

The CDC is now suggesting that women thinking of getting pregnant, and who may be exposed to the Zika virus through travel or because of where they live, should consider having their blood tested for Zika antibodies before they get pregnant. Having a baseline reading would help to interpret Zika tests done during a later pregnancy.

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The CDC is also suggesting that pregnant women who have no symptoms of Zika infection but may have been infected should be tested with two different tests at various stages of their pregnancies.

The problem is that a type of antibody triggered by Zika infection — it’s called IgM — can stick around in the blood for months, emerging evidence suggests.

In the case of dengue, a related virus, an IgM test can be positive for six months. And IgM responses were detected in the blood of some people more than a year after they were infected with West Nile, another related virus.

In the early days of the Zika epidemic, an IgM test that came back positive for the virus could pretty confidently be interpreted as a sign of recent infection, Dr. Henry Walke, the CDC’s incident manager for Zika told STAT. But now that the virus has been circulating for a while, that assumption can no longer be made in some cases.

For most people, it wouldn’t matter much. But for a woman being tested, as the CDC recommends, in the course of a pregnancy, knowing if the infection happened before conception — and poses no risk — or during the pregnancy is important information.

The problems with IgM testing are particularly acute in places where multiple flaviviruses — the family Zika and dengue viruses belong to — circulate, such as Puerto Rico. That’s because the test has a hard time differentiating between IgM triggered by these related viruses.

The CDC alert noted that there was little dengue transmission in Puerto Rico during 2016, so an IgM test for Zika that came back positive was likely a true indication of Zika infection. “However, if dengue virus transmission were to increase, guidance for interpretation of Zika virus IgM testing results may need to be reconsidered,” the CDC said.

Previously, the CDC had suggested that pregnant women who are asymptomatic for Zika but who could have been infected be tested using the IgM test in both the first and second trimester of pregnancy.

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Now the agency is telling doctors that, in addition, they should test those women once per trimester using a different test, one that looks in the blood for the genetic material of the Zika virus, called nucleic acid.

Walke admitted it’s not yet clear how long Zika nucleic acid is found in the blood after infection — in other words, how wide the testing window is. But it is known Zika nucleic acid declines over time, so a negative genetic test isn’t proof that infection didn’t occur. Pregnant women should be counseled on the limitations of these tests, the CDC guidance advises.

The new guidance also suggests that if an amniocentesis test is done, doctors should consider testing the fluid using the Zika nucleic acid test. Likewise, pregnant women who develop symptoms of Zika or who have a sexual partner who tests positive for Zika should be promptly tested using the nucleic acid test.

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