When it comes to Zika testing, urine is better than blood, according to new guidelines issued by the Centers for Disease Control and Prevention (CDC). The CDC recently updated its interim diagnostic testing guidance for the virus to recommend real-time reverse transcription-polymerase chain reaction (rRT-PCR) testing for urine specimens obtained two weeks after the illness starts.
rRT-PCR testing is usually the preferred test for Zika. But the virus' RNA is almost undetectable in blood samples after the first week of illness, according to a report published in the CDC's Morbidity and Mortality Report.
Study authors ran tests on multiple kinds of samples to figure out "the most sensitive and efficient testing algorithm for acute Zika virus disease," Dr. Andrea Bingham, vector-borne disease surveillance coordinator at the Florida Department of Health, and her colleagues wrote in the report. The team specifically looked at urine, blood, semen and saliva samples to see which would present the most accurate results.
Researchers found that urine samples were the best at flagging Zika, with 93% of specimens collected within 20 days of symptoms first appearing showing positive results. Urine specimens also displayed positive results 82% of the time when samples were collected more than 5 days after symptoms cropped up.
Urine samples were positive 95% of the time when the specimens were obtained the same day or within 5 days of illness onset. This compares to 56% for blood samples, according to the report.
"(The) testing results suggest urine might be the preferred sample type to diagnose acute Zika virus disease," the authors wrote in the report.
The CDC still recommends testing blood samples in addition to urine samples for specimens obtained less than 7 days after the onset of illness.
The jury is out on urine specimens collected more than two weeks after the symptoms appear, the agency said in its guidance. The CDC is not changing its recommendations for testing for this time frame and will continue to review its stance as new data surfaces, it said.