Ascertaining fetal Zika virus infection based on IgM antibody test in endemic settings
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First, the authors' rationale to include a Zika virus (ZIKV)‐specific IgM antibody test as part of the positive case definition, despite the test's cross‐reactivity with other Flavivirus infections and immunizations, is not clear. As described previously2, the levels of anti‐ZIKV IgM may appear elevated as a result of antibody cross‐reaction among viruses of the same family. A false‐positive result could therefore lead to misdiagnosis of an otherwise normal fetus. The International Society of Ultrasound in Obstetrics and Gynecology recommends the use of reverse transcription polymerase chain reaction (RT‐PCR) or consultation with an expert when interpreting ZIKV IgM positive results3. For example, studies in endemic populations in Colombia and Brazil did not include detection of ZIKV IgM antibodies for defining a positive case and instead used only RT‐PCR assays4, 5. Consequently, suggesting that a test for ZIKV IgM antibody alone is sufficient for diagnosing ZIKV would affect outcomes of clinical practice in endemic settings.