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Rapid testing for West Nile virus (WNV) RNA
An adjunctive test to serology for detection of early WNV infection
West Nile virus (WNV) is a mosquito-borne flavivirus (single-stranded RNA virus) that primarily infects birds, but occasionally infects horses and humans. Until the virus infection was recognized in 1999 in birds in New York City, WNV had been detected only in the Eastern hemisphere, with a wide distribution in Africa, Asia, the Middle East, and Europe. Most people who are infected with WNV do not experience symptoms. It is estimated that about 20% of those who become infected will develop West Nile fever with mild symptoms including headache, myalgia, and occasionally a skin rash on the trunk of the body. About 1 of 150 WNV infections (<1%) result in meningitis or encephalitis. Case fatality rates among patients hospitalized during recent outbreaks have ranged from 4% to 14%. Advanced age is the most important risk factor for death, and patients older than 70 years of age are at particularly high risk.
Laboratory diagnosis is best achieved by demonstration of specific IgG- and IgM-class antibodies in serum specimens. PCR testing can detect WNV RNA in plasma specimens from patients with recent WNV infection (ie, 3-5 days following infection) when specific antibodies to the virus are not yet present. However, the likelihood of detection is relatively low as the sensitivity of PCR detection is approximately 55% in cerebrospinal fluid and approximately 10% in blood from patients with known WNV infection.
A positive result indicates the presence of West Nile virus (WNV) RNA and is consistent with early WNV infection.
This assay should not be used for screening asymptomatic individuals, and should only be used to test patients with signs and symptoms of West Nile virus (WNV) disease.
The sensitivity of the assay is very dependent upon the quality of the specimen submitted.
A negative test does not exclude infection with WNV. Therefore, the results obtained should be used in conjunction with clinical findings and serologic test results to make an accurate diagnosis.
This assay detects both viable and nonviable virus. Test performance depends on viral load in the specimen and may not correlate with cell culture performed on the same specimen.
Possible cross reactivity with other flaviviruses (eg, dengue virus, St. Louis encephalitis, and Japanese encephalitis virus) may occur.
The following validation data supports the use of this assay for clinical testing.
Accuracy/Diagnostic Sensitivity and Specificity:
To determine the ability of the assay to detect RNA from clinical specimens, 30-negative specimens per specimen type (120 total: whole blood, serum, plasma, and CSF specimens) were spiked with whole West Nile virus (WNV) near the limit of detection (approximately 10 targets/microliter). The specimens were run in a blinded manner along with 30 negative (nonspiked) specimens for each matrix. All spiked specimens (100%) were positive and 100% of the nonspiked specimens were negative.
To supplement the above data, blinded proficiency panels were tested with this assay and demonstrated 100% concordance. In addition, 8 patient samples that were positive for West Nile virus RNA by this assay were tested with an alternative reference laboratory assay. Results showed 75% concordance.
Analytical Sensitivity/Limit of Detection (LoD):
The lower LoD of this assay is less than 10 targets/microliter.
Interassay precision is 100% and intraassay precision is 100%.
A total of 50 CSF specimens from normal donors were tested and found to be negative for targeted WNV RNA.
This is a qualitative assay and the results are reported as either negative or positive for targeted WNV.
1. Petersen LR, Brault AC, Nasci RS: West Nile Virus: Review of the Literature. JAMA 2013;310(3):308-315
2. Busch MP, Tobler LH, Saldanha J, et al: Analytical and clinical sensitivity of West Nile virus RNA screening and supplemental assays available in 2003. Transfusion 2005;5(4):492-499
3. New York City Department of Health. West Nile surveillance and control: an update for healthcare providers in New York City. City Health Information. June 2001;20(2)