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Rapid testing for West Nile virus (WNV) RNA
As an adjunct in the diagnosis of early WNV virus 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 was found only in the Eastern hemisphere, with a wide distribution in Africa, Asia, the Middle East, and Europe.(1-3) 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 or cerebrospinal fluid (CSF) specimens (WNV / West Nile Virus [WNV] Antibody, IgG and IgM, Serum or WNVC / West Nile Virus [WNV] Antibody, IgG and IgM, Spinal Fluid).
The specific identification of WNV by detection of IgM in CSF is the recommended test to document central nervous system disease, but this test may be falsely negative in CSF collected <8 days after the onset of symptoms. Alternatively, experiences in nucleic acid testing for WNV RNA in blood prior to transfusion have indicated that PCR can detect viremic target RNA from patients with known West Nile infection when specific antibodies to the virus are not present (ie, from 2-8 days after onset of symptoms).(4,5)
The likelihood of detection of West Nile virus RNA by PCR is relatively low. In cerebrospinal fluid, the clinical sensitivity is approximately 55%, and in blood, about 10%. Specificity of the assay in either matrix is approximately 100%.(6)
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 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 virus, 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 whole blood, serum, plasma, and cerebrospinal fluid (CSF) samples (120 total) were spiked with West Nile virus (WNV)-positive control material at 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. 100% of the specimens 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 WNV RNA by this assay were tested with an alternative reference lab assay. Results showed 75% concordance.
Analytical Sensitivity/Limit of Detection (LoD):
The LoD of this assay is approximately 10 targets/microliter.
Interassay precision is 100% and intraassay precision is 100%.
Fifty CSF samples 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.