Federal health officials are now recommending that people suspected of having Zika infections get tested for evidence of the virus in their urine in addition to undergoing blood tests.
The shift comes as new data show that traces of the virus can remain longer in urine than in blood, making for more accurate diagnosis. That’s especially important for pregnant women given the serious risk Zika infection poses to fetal brain development.
On the basis of the new data, the Centers for Disease Control and Prevention is advising clinicians to test both blood and urine within the first two weeks of symptoms.
With this additional testing, “there may be some infections that are more definitively diagnosed than they would have been,” CDC epidemiologist Marc Fischer said Tuesday. “This gives you a more specific finding of the presence of genetic material of the virus.”
In most people, Zika doesn’t even produce symptoms. Only one out of five infected individuals experiences the most common symptoms: fever, joint pain, rash and conjunctivitis (pink eye).
The CDC recommends that pregnant women with possible exposure to the virus be closely monitored and tested. Until now, initial diagnostics have been blood tests.
Zika usually remains in the blood of an infected person for about a week; after that, making a diagnosis can be more difficult. Other tests can look for the presence of antibodies that humans produce in response to the virus. But people who have been previously infected with dengue or other viruses related to Zika also produce antibodies. Tests to differentiate between those antibodies take longer and are more complicated.
In a report released Tuesday, the CDC said an analysis of blood and urine tests conducted by Florida’s health department found rates of virus detection from urine were higher than from blood. Florida, with 95 confirmed cases, is the state with the most number of Zika infections on the U.S. mainland.
Officials detected the virus in urine as early as the first day of illness and as late as 20 days after.
As of April 20, Florida health officials had collected urine specimens from 70 people infected with Zika. Overall, 65 of those had urine samples that tested positive for the virus. Among 55 people with urine and blood samples collected within the first five days of illness, 52 tested positive for Zika in their urine, compared to only 31 with a positive blood test.
More than five days after their symptoms began, nine out of 11 people tested positive for Zika in their urine. None of their blood samples signaled the virus.