False Positives Common With LabCorp Zika Testing, FDA Warns

December 23, 2016

Clinicians running a preliminary Zika virus test through LabCorp for their pregnant patients shouldn't jump to conclusions about the results, especially in light of a higher-than-expected rate of false positives, the US Food and Drug Administration (FDA) announced today.

Because the Zika test in question — called ZIKV Detect — is presumptive, it should always be confirmed by the Centers for Disease Control and Prevention (CDC) or by qualified laboratories. Therefore, clinicians should not rely on ZIKV Detect results as the only basis of "significant patient management decisions," the FDA said in a MedWatch safety communication. Likewise, pregnant women should not make healthcare decisions on the basis of partial information.

The need for caution is obvious because the Zika virus can cause catastrophic birth defects, such as microcephaly, as well as loss of pregnancy.

Confirmatory testing may take up to a month to perform, but it can be sped up if the CDC knows that the sample came from a pregnant woman. Clinicians should tell the laboratory conducting the presumptive ZIKV Detect test whether the patient is a pregnant woman or not so it can prioritize confirmatory testing by the CDC or a qualified laboratory.

ZIKV Detect, manufactured by InBios International, is designed to spot Zika virus IgM antibodies in human sera collected from patients with either a history of Zika infection, signs and symptoms, or a history of travel or residence in an active Zika transmission zone. The FDA approved ZIKV Detect in August. It is the first commercially available serologic IgM test for Zika. LabCorp and other commercial laboratories have switched to ZIKV Detect from a CDC assay authorized at the start of 2016.

Confirmatory testing is required for both the CDC assay and ZIKV Detect, and in the past, most presumptive positive results from each have proven accurate. However, that has not been the case with results of ZIKV Detect testing conducted by LabCorp, the FDA said. The CDC reported that the confirmation rate of presumptive positive results captured by LabCorp through ZIKV Detect is less than 50%.

The FDA said that it has not determined whether the high rate of false positives is related to ZIKV Detect itself or LabCorp. Tests for Zika IgM antibodies, the agency noted, remain useful for ruling out Zika exposure, but they always require confirmation.

More information on today's FDA announcement is available on the agency website.

To report any problems with ZIKV Detect, contact MedWatch, the FDA's safety information and adverse event reporting program, by telephone at 1-800-FDA-1088; by fax at 1-800-FDA-0178; online at https://www.accessdata.fda.gov/scripts/medwatch/medwatch-online.htm; with postage-paid FDA form 3500, available at https://www.fda.gov/MedWatch/getforms.htm; or by mail to MedWatch, 5600 Fishers Lane, Rockville, Maryland 20852-9787.

Follow Robert Lowes on Twitter @LowesRobert


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