As concern about Zika virus mounts, the Food and Drug Administration on Feb. 26 authorized use of an investigational test from the Centers for Disease Control and Prevention to detect antibodies in sera and cerebrospinal fluid.
Known as the Zika Immunoglobulin M Antibody Capture Enzyme-Linked Immunosorbent Assay (Zika MAC-ELISA), the test is available under an FDA Emergency Use Authorization to labs certified by CDC for high-complexity tests.
If infection is suspected, “the Zika MAC-ELISA may be ordered. Please contact your state or local health department to facilitate testing,” FDA said in a fact sheet for health care providers.
“At this time, there are no FDA approved/cleared tests available that can detect Zika virus in clinical specimens in the United States. Therefore, CDC has developed this test to detect evidence of Zika virus infections.” Positive and inconclusive results must be confirmed by CDC or authorized laboratories, the FDA noted.
Testing is appropriate in patients with signs and symptoms of Zika virus infection who recently traveled to areas with active transmissions. Anti-Zika IgM can usually be detected after about 4 days of symptoms, and remains detectable for about 3 months.
Most who are infected with the Zika virus don’t know they have it. For others, symptoms set in after a few days, tend to be mild, last about a week, and can include fever, rash, joint pain, and conjunctivitis. The virus has, however, been associated in Brazil with Guillain-Barre syndrome and microcephaly, although detection in pregnant women does not necessarily mean the fetus has been harmed.
So far, there have been more than 90 confirmed cases of Zika infection in the United States, most, but not all, in people who had recently traveled to endemic areas.
The FDA did not report sensitivity and specificity figures, but it’s clear from what it did say that reliability is an issue. Closely-related flavivirus infections, such as dengue fever, can trigger false positives, and in patients vaccinated against yellow fever or Japanese encephalitis, cross-reactive antibodies can also “make it difficult to identify which flavivirus is causing the patient’s current illness,” the fact sheet noted.
False negatives should be considered when “recent exposures or clinical presentation are consistent with Zika virus infection and diagnostic tests for other causes of illness are negative. Conversely, a negative result in an asymptomatic patient with a lower likelihood of exposure (e.g., a short term traveler to an affected area) may suggest the patient is not infected,” the FDA noted.
Zika MAC-ELISA labeling, fact sheets for patients, and other materials are available on the FDA’s Emergency Use Authorizations webpage. The CDC has a Zika virus site for health care providers, as well.