Symptoms due to Zika virus infections are usually mild tends to be mild, the initial symptoms can escape notice, lessening the opportunity to take a sample. Although the viremic period still has not been fully established, viral RNA has been detected in serum up to day 10 after the onset of symptoms. ZIKV RNA also has been detected in urine over an extended period in the acute phase, which means that could be an alternative sample to be considered.
Zika virus can be identified by RT-PCR in ill patients and from day 5 post onset of fever by serology through detection of specific IgM antibodies. Serological cross-reactions with closely related flaviviruses such as Dengue fever are possible.
ZIKV-specific IgM antibodies can be detected by ELISA or immunofluorescence assays in serum specimens from day 5 after the onset of symptoms. Since a single serum in the acute phase is presumptive, it is recommended by the WHO that a second sample be taken 1–2 weeks after the first sample to demonstrate seroconversion (negative to positive) or a fourfold increase on the antibody titer (with a quantitative test).
The interpretation of the serological tests is especially important for the diagnosis of ZIKV. In primary infections (first infection with a flavivirus) it has been demonstrated that antibodies cross-reaction is minimal with other genetically related viruses. However, it has been demonstrated that sera of individuals with a previous history of infection from other flaviviruses (especially dengue, yellow fever and West Nile) can cross-react in these tests. Although neutralization by plaque reduction (PRNT) offers a greater specificity in the detection of neutralizing antibodies (IgG), cross-reactions have also been documented; in fact, some patients with a previous history of infection by other flaviviruses have shown up to a fourfold increase in neutralizing antibody titers when infected with ZIKV.