Zika Virus Test Interpretation Guidelines
Zika virus is a single-stranded RNA virus in the genus Flavivirus and is closely related to dengue, West Nile, Japanese encephalitis, and yellow fever viruses. Zika and dengue virus have similar geographic distribution and transmission cycles. Most infections are spread by mosquitoes although the virus can also be transmitted sexually and through the blood. They cause similar symptoms in infected patients.
Many people do not develop symptoms after infection with Zika virus. Those who do, exhibit symptoms similar to other flavivirus infections that include fever, skin rashes, conjunctivitis, muscle and joint pain, malaise, and headache. These symptoms are usually mild and last for 2-7 days. Rarely, some individuals develop Guillain-Barré syndrome, which can cause temporary paralysis.
Viremia occurs several days before illness onset until a week afterwards. Zika virus IgM antibodies develop during the first week of illness. There is limited data on the duration of Zika virus IgM antibody following infection, but studies on patients with West Nile virus have shown that serum IgM antibodies can persist at least one year after disease onset. Neutralizing IgG antibodies to Zika virus develop shortly after IgM antibodies. Neutralizing antibodies usually persist for many years after flavivirus infections and may confer prolonged immunity. Neutralizing antibody titer against a flavivirus to which a person was previously exposed or vaccinated against might be higher than the titer against a virus causing a recent infection.
Diagnostic testing for Zika virus infection includes both molecular and serologic methods. FDA has issued an Emergency Use Authorization (EUA) to authorize the use of the Centers for Disease Control and Prevention’s (CDC) Zika IgM antibody capture ELISA (Zika MAC-ELISA) for the in vitro qualitative detection of human IgM antibodies to Zika virus in serum or cerebrospinal fluid. FDA has also authorized use of CDC’s Trioplex Real-time RT-PCR Assay (Trioplex rRT-PCR) for the in vitro qualitative detection of Zika virus in serum, cerebrospinal fluid amniotic fluid and urine. This assay detects Zika virus, dengue virus and chikungunya virus.
On April 28, 2016, Quest Diagnostics announced that it had received FDA EUA for a Zika Virus RNA Qualitative Real-Time RT-PCR test developed by its subsidiary, Focus Diagnostics. This is the first test from a commercial clinical laboratory to receive EUA. This test can be performed at any CLIA high-complexity laboratory in the Quest Diagnostics network, which includes several dozen labs in the United States, including in Toa Baja and Puerto Rico Quest Diagnostics plans to make the new test broadly available to physicians for patient testing.
For persons with suspected Zika virus disease, a positive rRT-PCR result confirms Zika virus infection, and no antibody testing is indicated. Because viremia declines over time, a negative rRT-PCR result does not necessarily exclude Zika virus infection. In these cases, IgM ELISA and confirmatory neutralizing antibody testing can detect recent Zika virus infections.
Serum IgM antibody testing for Zika and dengue virus infections should be performed if rRT-PCR is negative. For serum specimens collected less than 7 days after onset of symptoms, the combination of a negative rRT-PCR result and negative IgM antibody testing suggests that recent infection is unlikely. However, a negative IgM antibody test, in the absence of rRT-PCR testing, might reflect specimen collection before development of detectable antibodies and does not rule out infection. For specimens collected from 7 days to 12 weeks after onset of symptoms, a negative IgM antibody result to both Zika and dengue viruses rules out recent infection with either virus.
If either the Zika or dengue virus IgM antibody testing yields positive, equivocal, or inconclusive results, plaque reduction neutralization test (PRNT) against Zika and dengue viruses should be performed. PRNT is a confirmatory test that measures virus-specific neutralizing antibody titers to identify the infecting virus and rule out false positive ELISA results. Without PRNT it is not possible to determine if a positive IgM ELISA result against Zika virus indicates recent infection or a false positive result. A PRNT titer of 10 or higher against Zika virus, together with negative PRNT titers against other flaviviruses confirms the presence of a recent Zika virus infection. A PRNT titer of 10 or higher for both Zika and another flavivirus indicates a recent flavivirus infection but cannot determine the specific virus. A negative PRNT against Zika virus in a specimen that is collected more than 7 days after illness onset rules out Zika virus infection. For specimens collected less than 7 days after onset of symptoms, the combination of a negative rRT-PCR and a PRNT titer less than 10 suggests that Zika virus infection did not occur. In the absence of rRT-PCR testing, a PRNT titer less than10 by itself may reflect specimen collection before development of detectable neutralizing antibodies and does not rule out infection.
For asymptomatic pregnant women residing in an area with local Zika virus transmission, IgM testing should be performed upon initiation of prenatal care, mid-second trimester, and if any fetal abnormalities are detected during ultrasound evaluation. For asymptomatic pregnant women with a history of travel to endemic Zika virus areas, antibody testing should be performed on specimens collected 2–12 weeks post travel. Results are interpreted as for symptomatic persons. Specimens collected more than 12 weeks after travel may give falsely negative results. PRNT testing for women in this group is not recommended because any result other than a PRNT titer of less than 10 for Zika virus could represent previous infection or vaccination against a flavivirus.
Interim Guidance for Interpretation of Zika Virus Antibody Test results, MMWR, June 3, 2016; 65(21)
http://www.cdc.gov/mmwr/volumes/65/wr/mm6521e1.htm?s_cid=mm6521e1_e
