Zika: To Test or Not to Test?

CDC Guidance for Zika Virus Testing

Pregnancy and Birth Defects Task Force and the Epidemiology Task Force, CDC's 2016 Zika Virus Response


November 17, 2016

Editorial Collaboration

Medscape &

Zika Testing

As the number of laboratory-confirmed Zika virus disease cases has increased in the United States and abroad, so have efforts to establish effective ways to rapidly screen and test for Zika virus. This review provides clinicians with information about Zika virus screening, testing, and interpretation of results based on Centers for Disease Control and Prevention (CDC) guidance. CDC provides interim guidance for healthcare providers, but healthcare providers should consult their state, tribal, local, and territorial health departments for testing approval and coordination.

Overview of Diagnostic Tests

Several discrete tests are available to clinicians who wish to refer patients for Zika virus testing in accordance with CDC guidance. There are no licensed Zika virus tests in the United States. Rather, public health and commercial laboratories use assays approved for emergency use by the US Food and Drug Administration. This mechanism, called an Emergency Use Authorization, allows unlicensed tests to be used in a public health emergency.

Diagnostic testing for Zika virus infection is challenging but can be accomplished using both molecular and serologic methods. Real-time reverse transcription polymerase chain reaction (rRT-PCR) is the molecular method. It detects viral RNA in body fluids (such as serum, whole blood, urine, cerebrospinal fluid) or tissues (such as placenta). The detection of RNA provides a definitive diagnosis of recent Zika virus infection.

Serologic tests include the anti-Zika virus immunoglobulin M (IgM) antibody capture enzyme-linked immunosorbent assay (MAC-ELISA), and the plaque reduction neutralization test (PRNT). Zika virus MAC-ELISA is used to detect anti-Zika virus IgM antibodies in serum or cerebrospinal fluid. PRNT is used to measure virus-specific neutralizing antibody titers.

Zika virus RNA is relatively short-lived, typically persisting in serum and urine for 14 days or less. However, there may be some individualized differences. Emerging data indicate that Zika virus RNA persists in some pregnant women longer than the 1-2 weeks previously reported.

Because viral RNA levels decline over time, a negative result does not rule out Zika virus infection. Therefore, serum anti-Zika IgM antibody testing should be performed. Anti-Zika virus IgM develops during the first week of illness and can be detected up to 12 weeks after infection, but it can persist longer in some cases. If anti-Zika virus IgM is detected, PRNT should be performed to confirm this result as indicated by CDC laboratory guidance.

Several limitations should be kept in mind when interpreting both Zika virus molecular and serologic assays. First, because the timeframe when Zika virus RNA remains detectable in body fluid is short, negative results do not rule out infection. Second, testing for anti-Zika virus IgM can result in false-positive results because of significant cross-reactivity with other flaviviruses (such as dengue), and also for other nonspecific reasons. Third, PRNT may not distinguish the type of infecting virus in people previously infected with or vaccinated against a related flavivirus.