Anti-WNV IgM

Useful For


Presumptive diagnosis of West Nile virus infection


Reflex Tests


Reporting Name

Available Separately

Always Permormed

WNV Antibody IgG YES YES
WNV Antibody IgM YES YES
Plaque Reduction Neutralization Test (PRNT) NO NO



Testing Algorithm


WNV IgG and / or IgM reactive specimens will be confirmed by Plaque Reduction Neutralization Test (PRNT).


Indications for Testing


Aid in the differential diagnosis of meningoencephalitis presenting during mosquito season (in Newfoundland & Labrador mosquito season is typically June through December), otherwise in returning travelers from endemic countries.


This assay is not intended as a screening tool and should only be interpreted in the context of clinically established meningoencephalitis.


Special Instructions and Forms


Completion of relevant patient/travel history on laboratory requisition is mandatory for testing to proceed.


Clinical Information


Most people who are infected with WNV will not have any type 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. 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 can detect WNV RNA in specimens from patients with WNV infection when specific antibodies to the virus are not 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.


West Nile virus (WNV) is a mosquito-borne flavivirus (single-stranded RNA) that primarily infects birds but occasionally infects horses and humans. 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 2002, a total of 3,389 human cases of WNV infection were reported from 37 states (794 cases in Illinois); 2,354 (69%) presented with meningoencephalitis, 704 (21%) had West Nile fever, and 331 (10%) had an unspecified illness. Overall, the WNV epidemic in the United States was the largest arboviral meningoencephalitis outbreak documented in the Western hemisphere. In addition, 33 cases of probable WNV infection occurred among persons who had received blood components in the month before illness onset.


Reference Values






Anti-WNV IgM


REACTIVE: Presence of specific IgM class antibodies in a serum specimen is consistent with current or recent infection with West Nile virus (WNV) or another flavivirus (due to antigenic cross-reactivity). WNV IgM REACTIVE results in children should be interpreted with caution as enterovirus cross-reactivity may cause false-positive findings. By the 8th day of illness, most infected persons will have detectable serum IgM antibody to WNV; in most cases it will be detectable for at least 1 to 2 months after onset of illness, in some cases it will be detectable for > 500 days post exposure.


NON-REACTIVE: Absence of IgM class antibodies to WNV is consistent with lack of acute-phase infection with this virus. Specimens drawn too early in the acute phase (eg, before 8 days postinfection) may be negative for IgM-specific antibodies to WNV. If WNV infection is suspected, a second specimen drawn approximately 7 – 14 days postinfection should be tested.


INDETERMINATE: The specimen tested near the cut-off. Immediate recollection and repeat testing should clarify the serostatus.


Clinical Reference


Focus Diagnostics. 2004. West Nile Virus IgM DxSelectâ„¢, Enzyme-linked Immunosorbent Assay (ELISA): package insert. Focus Diagnostics, Cypress, CA.


Lanciotti, R. S., and Tsai, T. F. 2007. Arboviruses, p. 1486-1500. In Murray, P. R., Baron, E. J., Jorgensen, J. H., Landry, M. L., and Pfaller, M. A. Manual of Clinical Microbiology, 9th ed., vol. 2. ASM Press, American Society for Microbiology, Washington, DC.


Vaughn, D. W., Barrett, A., and Solomon, T. 2010. Flaviviruses (Yellow Fever, Dengue, Dengue Hemorrhagic Fever, Japanese Encephalitis, West Nile Encephalitis, St. Louis Encephalitis, Tick-Borne Encephalitis), p. 2133-2156. In Mandell, D., Bennett, J. E., and Dolin, R. Principles and practice of infectious diseases, 7th ed., vol. 2. Churchill Livingstone, Elsevier, Philadelphia, PA.


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