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West Nile Virus Antibody IgG
West Nile Virus Antibody IgM
Test results should be used in conjunction with clinical evaluation, exposure history, and other available diagnostic procedures.
The significance of negative test results in immunosuppressed patients is uncertain.
False-negative results due to competition by high levels of IgG, while theoretically possible, have not been observed.
False-positive results may occur in patients infected with other flaviviruses, including dengue virus, St. Louis virus, and Zika virus and in persons previously infected with West Nile virus (WNV).
Because closely related arboviruses exhibit serologic cross-reactivity, it sometimes may be epidemiologically important to attempt to pinpoint the infecting virus by conducting plaque reduction neutralization tests (PRNT) using an appropriate battery of closely related viruses. Such testing is available through the Centers of Disease Control and Prevention and select public health laboratories.
WNV antibody results for cerebrospinal fluid (CSF) should be interpreted with caution. Complicating factors include low antibody levels found in CSF, passive transfer of antibody from blood, and contamination via a traumatic lumbar puncture.
West Nile virus (WNV) is a mosquito-borne flavivirus (single-stranded RNA) that primarily infects birds but can also infect humans and horses. WNV was first isolated in 1937 from an infected person in the West Nile district of Uganda. Until the viral infection was recognized in 1999 in birds in New York City, WNV was found only in the Eastern Hemisphere, with wide distribution in Africa, Asia, the Middle East, and Europe. In 2012, a total of 5674 cases of WNV were reported to the Centers for Disease Control and Prevention (CDC), among which 2873 (51%) were classified as neuroinvasive disease (eg, meningitis or encephalitis) and 286 (5%) cases resulted in death.
Most people who are infected with WNV will not develop clinical signs of illness. It is estimated that about 20% of those who become infected will develop West Nile fever with mild symptoms, including fever, headache, myalgia, and occasionally a skin rash on the trunk of the body. 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. Polymerase chain reaction (PCR) (LCWNV / West Nile Virus, Molecular Detection, PCR, Spinal Fluid) can detect WNV RNA in 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.
Presence of specific IgM-class antibodies to West Nile virus (WNV) is consistent with the acute phase of WNV meningitis or encephalitis. In the very early stages of acute WNV infection, IgM may be detectable in cerebrospinal fluid (CSF) before it becomes detectable in serum.
The absence of IgM antibodies to WNV may indicate absence of disease. However, specimens collected too early in the acute phase may be negative for IgM-class antibodies to WNV. If WNV central nervous system infection is suspected, a second specimen should be collected in 1 to 2 weeks and tested.
The presence of IgG-class antibodies to WNV may indicate recent or past central nervous system (CNS) infection with WNV. Clinical correlation is necessary.
This assay is unable to distinguish between intrathecal antibody synthesis and serum antibodies introduced into the CSF at the time of lumbar puncture or from a breakdown in the blood-brain barrier. Positive results should be interpreted with other laboratory and clinical data prior to a diagnosis of CNS infection.