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FDA Warns of False Positives for Zika Test

NEW YORK (360Dx) – The US Food and Drug Administration said today that Laboratory Corporation of America reported false-positive results using the first commercially available Zika serological IgM test, which is manufactured by InBios International.

As a result, the FDA issued recommendations to physicians using the test not to depend solely on it to diagnose patients.

The Centers for Disease Control and Prevention confirmed less than half of the results that were presumed positive by LabCorp using the test, FDA said.

LabCorp captured the results while conducting testing with the InBios ZIKV Detect IgM Capture ELISA test. Commercial labs use the assay for preliminary detection of Zika virus IgM antibodies in human sera. Clinicians collect samples from patients with a history of clinical signs and symptoms associated with Zika virus infection, or a history of residence in or travel to a geographic region with active Zika transmission at the time of travel, or both.

In its alert to physicians caring for pregnant women meeting these criteria, the agency said that confirmatory tests could normally take from a week to a month to complete, but the FDA urged healthcare practitioners to let the CDC and qualified laboratories know when samples come from pregnant women so that the labs can prioritize testing that confirms positive results. 

"While the FDA has not yet determined if the reported false positives are related to the ZIKV Detect test or the [LabCorp] commercial testing facility, it is important to remember that IgM tests remain useful in ruling out Zika exposure but require confirmatory testing," the FDA said in a statement.

The agency had given approval in August for use of the InBios serological IgM test as the first commercially available Dx for Zika. FDA noted that several commercial laboratories started migrating serological testing to the commercial assay from a CDC assay that it had authorized at the beginning of this year.

Based on past performance, "most of the presumptive positive results from both tests have been ultimately confirmed," FDA said, adding that the CDC test and the ZIKV Detect test report "only presumptive positive results, and samples have to be sent for confirmation" by CDC or qualified laboratories.

After transitioning to the commercial test, LabCorp observed higher than expected false positive results, FDA added.