What are the diagnostic tests for West Nile Virus (WNV) infection?

From the Guidelines

Testing for West Nile virus should be done by detecting virus-specific IgM- and/or IgG-class antibodies in serum and/or CSF, as recommended by the Infectious Diseases Society of America and the American Society for Microbiology 1. When testing for West Nile virus, it is essential to consider the timing of symptom onset, as IgM antibodies are typically detectable 3–8 days after symptoms begin 1.

• The presence of anti-WNV IgM antibodies in serum and/or CSF is indicative of a recent WNV infection.
• Seroconversion to anti-WNV IgM and/or IgG positivity between acute and convalescent sera (collected 7–10 days apart) is strongly suggestive of a recent WNV infection 1.
• Detection of WNV RNA by NAAT in serum and CSF may be associated with higher sensitivity in immunosuppressed patients due to the delayed immune response and thus prolonged WNV viremia in this population 1. Key considerations in testing for West Nile virus include:
• False-positive results for both anti-WNV IgM and IgG antibodies may occur in patients who have been vaccinated against yellow fever virus or following natural infection with other flaviviruses (eg, dengue, St Louis encephalitis viruses) 1.
• To rule out cross-reactivity, it is recommended that specimens reactive for WNV antibodies be tested by PRNT 1. In clinical practice, a combination of serologic testing and NAAT may be necessary to confirm the diagnosis of West Nile virus infection, particularly in severe cases with neurological symptoms.

From the Research

Testing Methods for West Nile Virus

• The diagnosis of West Nile virus (WNV) can be challenging due to short-term and low-level viremia, flavivirus cross-reactivity, and long immunoglobulin M (IgM) persistence 2.
• Serologic testing, which detects IgM antibodies, is commonly used but may require cumbersome confirmatory testing and may fail to detect infection in specimens collected early in the course of illness 3.
• Molecular testing, such as reverse transcription-polymerase chain reaction (RT-PCR), can detect WNV RNA in serum, cerebrospinal fluid (CSF), and urine samples 4, 2, 5, 6.

Specimen Types for Testing

• Whole blood (WB) is considered a preferred specimen type for WNV PCR testing, especially for neuroinvasive disease, as it can detect WNV RNA in patients with negative serologic testing 4.
• Urine is also a valuable specimen type, as WNV is excreted in urine longer and at higher concentrations compared to blood, and urine qRT-PCR can confirm more cases than serum and CSF testing alone 2, 6.
• CSF PCR testing is also useful, especially if WB PCR is negative in patients with suspected neuroinvasive disease 4.

Timing of Sample Collection

• The timing of sample collection is crucial, as WNV RNA detection rates vary depending on the time of sampling 2.
• WNV RNA can be detected in urine and serum samples collected within 7 days of symptom onset, but the detection rate decreases over time 2.
• IgM antibodies can be detected in serum samples collected 7 days or more after symptom onset, but the frequency of positive IgM antibodies decreases over time 2.

Diagnostic Assays

• RT-PCR is a sensitive and specific assay for detecting WNV RNA in clinical samples 3, 4, 2, 5, 6.
• Reverse transcription polymerase spiral reaction (RT-PSR) assay is a rapid and real-time diagnostic tool for detecting WNV RNA in acute-phase serum samples 5.
• Urine qRT-PCR is a valuable diagnostic test for confirming probable cases of WNV neuroinvasive disease 6.