When interpreting and reporting Zika virus test results, there are some important considerations to keep in mind:
- Zika virus RNA may be detected in serum for approximately 4-7 days following the onset of symptoms. Therefore, the optimum time to perform serum RNA testing is during the first week after the onset of clinical illness. The persistence of Zika virus RNA detectable in urine is not well characterized, but may be longer than in serum. For patients who are 2-12 weeks post-symptom onset, serologic testing for Zika IgM should be considered. Test results should be used in conjunction with clinical signs and symptoms, epidemiological information, and relevant travel history to diagnose Zika virus infection.
- A positive test result for Zika virus RNA typically indicates that RNA from the Zika virus was detected in the patient’s sample. Laboratory test results should always be considered in the context of clinical observations and epidemiologic data in making a final diagnosis and patient management decisions. For guidelines on the Zika virus, please refer to http://www.cdc.gov/zika/hc-providers/index.html.
- In the event of a false positive result, risks to patients could include the impaired ability to detect and receive appropriate medical care for the true infection causing the symptoms and, in pregnant women, an unnecessary increase in the monitoring of a woman’s pregnancy.
- All positive Zika virus test results should be reported to local and state health authorities.
- It should be emphasized that the identification of Zika virus infection in a pregnant woman does not provide any definitive information about the state of health of the fetus. Many questions remain about the association between Zika virus infection in a mother and the impact on the fetus.
- A negative test for Zika virus in the specimen means that RNA from the Zika virus is not present in the specimen at the detection level of the assay. However, a negative result for the virus does not rule out infection with the virus and should not be used as the sole basis for treatment or other patient management decisions.
- A negative Trioplex rRT-PCR test result or a negative result from other NAT assays should not be interpreted as demonstrating that the patient has not had Zika virus infection. Negative rRT-PCR tests are known to occur in Zika infection, particularly if testing was conducted more than seven days after the onset of symptoms or in asymptomatic individuals. The possibility of a false negative result should especially be considered if the patient’s recent exposures or clinical presentation indicate Zika virus infection is likely, and diagnostic tests for other causes of illness are negative. If there is doubt about the accuracy of the symptom onset date or if the patient lacks symptoms, serological testing of negative serum specimens may be appropriate to look for evidence of infection. For CSF, amniotic fluid, and urine, it is especially important to note that these are not the primary diagnostic specimen types. Negative results in these specimen types do not necessarily mean that an individual is not infected. When negative results are obtained for these specimen types, attention should be directed to the result for the patient-matched serum specimen.
- Confirmation of positive and equivocal IgM results is necessary and should be conducted using the PRNT assay and according to the CDC's algorithm. If PRNT testing is indicated, that testing must be conducted by the CDC or by a PRNT laboratory, designated by the CDC to do confirmatory PRNT testing for the Zika response.
