Whole blood PCR testing preferable for West Nile virus detection

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Key takeaways:

  • Whole blood PCR testing was more effective than plasma for detecting West Nile virus.
  • Cerebrospinal fluid PCR tests should be ordered regardless of whole blood results if neuroinvasive disease is suspected.

PCR-based West Nile virus testing using whole blood was effective and favored compared with plasma testing as an initial test for infection, researchers reported.

“There was already evidence building that whole blood might be a better specimen type for PCR-based detection of West Nile virus,” Erin H. Graf, PhD, co-director of the Mayo Clinic microbiology laboratory, told Healio.


PCR-based whole blood testing for West Nile virus may be the preferable method of testing vs. PCR testing using plasma for initial infection testing. Image: Adobe Stock.

“The community (infectious disease physicians/practitioners and microbiologists) were all familiar with the limitations of PCR testing of plasma-poor sensitivity,” Graf said.

To assess the sensitivity of whole blood PCR testing for West Nile virus, the researchers preferentially “turned off” access to plasma testing in favor of whole blood PCR testing during the 2021 West Nile virus outbreak in Arizona.

During this time, they assessed ordering practices, test performance and patient characteristics of probable and confirmed cases among patients at the Mayo Clinic Arizona with positive test results for West Nile virus antibodies and/or nucleic acid between June and December 2021.

According to the study, they identified a total of 190 probable and confirmed cases, including 127 patients (66.8%) with neuroinvasive disease. Among the cases, only 29.5% had West Nile virus PCR testing ordered on whole blood, of which 80.3% were positive, including seven cases for which West Nile virus serologic testing was negative and five for which no serologic testing was ordered.

In comparison, the assessment showed that only 23.7% of cases that had cerebrospinal fluid PCR ordered had a positive result, including three cases that were negative by whole blood PCR, whereas West Nile virus PCR on whole blood detected 12 neuroinvasive cases that were cerebrospinal fluid PCR negative.

Based on these findings, Graf said that when West Nile virus is “on the differential for a patient,” whole blood PCR should be the recommended test.

“Antibody testing may be useful as a supplemental test, especially in later stages of infection,” Graf concluded. “Collection and PCR testing of cerebrospinal fluid may not be needed if whole blood is positive and the patient has a clinical diagnosis of neuroinvasive disease (imaging, symptoms, etc.) given the lack of sensitivity of PCR in cerebrospinal fluid.”

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