We aimed to determine adherence to West Nile virus (WNV) CSF testing guidelines and analyze factors contributing to improved adherence.

Diagnosis of WNV neuroinvasive disease requires appropriate testing given its nonspecific presentation. Centers for Disease Control and Prevention guidelines recommend CSF testing of WNV-specific immunoglobulin M (IgM) in all patients, with additional reverse transcription-polymerase chain reaction (RT-PCR) testing only in those who are unable to mount a humoral response due to significant immunosuppression. Studies report WNV testing underutilization, but factors impacting provider choice between the appropriate IgM test and lower-sensitivity RT-PCR test when both are available are not described.

We analyzed testing patterns for suspected neuroinvasive WNV in a retrospective analysis of 1304 adults in two Boston hospitals who underwent CSF WNV-specific IgM or WNV RT-PCR testing between 2016 and 2023. Both tests were available during the study’s duration. Relevant clinical, laboratory, and demographic data were extracted from the electronic health record.

The median age was 63 years, and 46% of patients were female. Sole testing with CSF RT-PCR occurred in 73% of patients, and WNV testing guidelines were followed in only 26% of cases. Elevated CSF protein, CSF lymphocytic pleocytosis, neurology service admission, immunocompetence, race, and hospital site were significantly associated with improved guideline adherence. WNV tests were positive in 26 patients, with only four RT-PCR positives in 1,133 tests. Patients whose CSF testing adhered to guidelines were 12 times more likely to receive a WNV diagnosis.

Diagnostic testing guidelines alone may not be sufficient to ensure appropriate CSF WNV IgM testing when RT-PCR testing, which is not recommended in immunocompetent patients, remains readily available. Differential guideline adherence based on institutional factors suggests that systems changes may be key in shaping provider behavior around appropriate diagnostic testing.