Interpretation

IgG:

The presence of IgG-class antibodies to West Nile virus (WNV) in serum indicates infection with WNV at some time in the past. By 3 weeks postinfection, virtually all infected persons should have developed IgG antibodies to WNV. If acute-phase infection is suspected, serum specimens drawn within approximately 7 days postinfection should be compared with a specimen drawn approximately 14 to 21 days after infection to demonstrate rising IgG antibody levels between the 2 serum specimens.

IgM:

Presence of specific IgM-class antibodies in a serum specimen is consistent with acute-phase infection with WNV. 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 following disease resolution and, in some cases, will be detectable for 12 months or longer.

The absence of IgM antibodies to WNV is consistent with lack of acute-phase infection with this virus. Specimens collected too early in the acute phase (eg, before 8-10 days postinfection) may be negative for IgM-specific antibodies to WNV. If WNV is suspected, a second specimen collected approximately 14 days postinfection should be tested.

In the very early stages of WNV infection, IgM may be detectable in cerebrospinal fluid before it becomes detectable in serum.