Urine More Accurate Than Blood for Zika Testing

By Brenda Goodman, MA
WebMD Health News

Reviewed by Brunilda Nazario, MD

May 10, 2016 -- Blood tests appear to miss about half of all recent Zika infections, according to a new study.

As a result, the CDC is changing its testing recommendations. They're advising doctors to check the urine and blood of patients who've had symptoms of the virus for less than 14 days. Previously, the agency recommended only blood tests.

In a recent study done by the Florida Department of Health, 66 people suspected of having Zika infections had their blood and urine tested on the same day.

The virus was detected in the urine of 52 out of 55 patients (95%) who were tested within 5 days of getting their first symptoms. Blood tests, meanwhile, picked up the virus in only 31 out of those same 55 patients (56%).

What's more, urine tests were able to pick up Zika for almost 2 weeks after symptoms first appeared, while blood tests only caught the virus within the first week.

Saliva was also more likely to flag the virus than blood, especially in the early days of infection. All patients tested within 2 days of their first symptoms tested positive for Zika virus in their saliva, while blood tests caught only about two-thirds of patients tested on the second day of symptoms.

The virus is mainly mosquito-borne, although some cases have been sexually transmitted.

One expert says the new study suggests that sex might not be the only way the infection is passed from person to person.

"It does raise that question," says William Schaffner, MD, an infectious disease expert at Vanderbilt University.

The test that picks up Zika during an active infection detects the virus by tagging a piece of its genetic code. But that test can't tell doctors whether the virus is infectious or not when it is spotted.

"It can't tell a live soldier from a dead one," Schaffner says.

But if the virus stays alive in saliva for days after the infection, it might mean that people could pass it to others through "intimacies that are not necessarily sexual," Schaffner says, like coughing or kissing.

"At the moment, that has not been documented," he says.

Spreading rapidly through the Caribbean and South America, the Zika virus causes microcephaly in babies born to infected pregnant women. Babies with microcephaly are born with smaller-than-normal heads and brain damage. The virus has also been linked to other health problems, although rare, in adults. Experts say 4 out of 5 people infected with Zika don't show symptoms.

Local transmission -- where mosquitoes become infected and pass the virus -- has not yet happened in the U.S. Cases have been seen in travelers returning from areas where Zika is spreading locally.

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SOURCES: William Schaffner, MD, professor of preventive medicine, Vanderbilt University, Nashville, TN. CDC, Morbidity and Mortality Weekly Report, May 10, 2016.

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