The Complex Interpretation and Management of Zika Virus Test Results

Kenneth W. Lin, MD, MPH; John D. Kraemer, JD, MPH; Rachael Piltch-Loeb, MSPH; Michael A. Stoto, PhD


J Am Board Fam Med. 2018;31(6):924-930. 

In This Article

Indications for Testing

Whether one is tested for Zika virus at all, what type of test is used, and when the test is done depend on the likelihood that one has been infected (eg, travel to an area of known transmission or contact with someone exposed) and the consequences of that infection (especially for children of women exposed during pregnancy). The type of test performed and timing relative to exposure depend on public health guidance, testing algorithms, test availability, and capacity. Whether and what type of testing is done also depends on where state and local health departments believe transmission is occurring.

Early in the US Zika virus outbreak, the only laboratories capable of testing samples were state and local health departments. Because testing capacity was limited, they (working in collaboration with the CDC) determined whether a sample was eligible to be tested by using algorithms that involve clinical criteria, especially (1) symptoms such as fever, rash, arthralgia, and conjunctivitis and associated conditions such as Guillain-Barré syndrome; (2) high-risk situations, such as pregnancy, and discovery of prenatal or neonatal outcomes, such as microcephaly or other specified conditions; (3) and "epidemiologic linkage," a combination of risk factors, including recent travel to areas with ongoing transmission; having sexual contact with partners who recently traveled to such an area; being a recipient of blood, blood products, or an organ transplant from a person with known infection; or clinical suspicion of mosquito-borne transmission. For example, Table 2 illustrates the criteria used by the New York City Health and Hospitals department in early 2016 to determine whether someone was eligible to be tested. Other jurisdictions likely followed the CDC's guidance less formally, taking into account local testing capacity.

Early in the outbreak, only those with known risks were eligible to be tested, so the pretest probability of infection likely was relatively high. As time progressed, testing became more available and lab capacity constraints were relaxed. In addition, private laboratories began to offer Zika virus testing through an emergency order from the Food and Drug Administration. Although the CDC recommends that clinicians contact their state or local health department to request Zika virus testing,[10] private laboratories can be accessed independently by providers and patients. Private labs and services, including Personal Labs, Quest Diagnostics, and LabCorp, offer both reverse transcription-PCR and ELISA Zika virus tests independently of state and local health departments. Patients may pay out of pocket for some private tests, with costs ranging from $500 to $3000 (LabCorp sales representative, personal communication).[18] Tests that are independently requested by patients are not covered by insurance, creating a potential bias toward those who have the availability to pay for a test (if test results are even being reported). There is wide state-by-state variation in private labs. Some states do not allow for private Zika virus testing. The cost, location, and availability of private tests vary, adding to the complexity of tracking potential Zika virus cases.

As a consequence of the increasing capacity, tests became more easily available to individuals who did not meet the CDC criteria, so, on average, the pretest probability of infection likely dropped. Because testing can be costly, its use likely varied by income, and because testing capacity varied geographically, the pretest probability likely varied from location to location as well.