Immunodiagnosis
Antibody Detection
Enzyme immunoassay (EIA) kits for Entomoeba histolytica antibody detection as well as EIA kits for antigen detection are commercially available in the United States.  Antibody detection is most useful in patients with extraintestinal disease (i.e., amebic liver abscess) when organisms are not generally found on stool examination.  Antigen detection may be useful as an adjunct to microscopic diagnosis in detecting parasites and can distinguish between pathogenic and nonpathogenic infections.

The indirect hemagglutination (IHA) test has been replaced by commercially available EIA test kits for routine serodiagnosis of amebiasis.  Antigen consists of a crude soluble extract of axenically cultured organisms.  The EIA test detects antibody specific for E. histolytica in approximately 95% of patients with extraintestinal amebiasis, 70% of patients with active intestinal infection, and 10% of asymptomatic persons who are passing cysts of E. histolytica.  If antibodies are not detectable in patients with an acute presentation of suspected amebic liver abscess, a second specimen should be drawn 7-10 days later.  If the second specimen does not show seroconversion, other agents should be considered.  Detectable E. histolytica-specific antibodies may persist for years after successful treatment, so the presence of antibodies does not necessarily indicate acute or current infection.  Specificity is 95% or higher: false-positive reactions rarely occur.  Although the immunodiffusion test is as specific, it is slightly less sensitive than the IHA and EIA and requires a minimum of 24 hours to obtain a result, in contrast to 2 hours required for the IHA or EIA tests.  However, the simplicity of the procedure makes it ideal for the laboratory that has only an occasional specimen to test.  The IHA and EIA tests are more suitable for laboratories that have frequent requests for amebiasis serology.  Although detection of IgM antibodies specific for E. histolytica has been reported, sensitivity is only about 64% in patients with current invasive disease.  Several commercial EIA kits for antibody detection are available in the United States.

Antigen Detection
Antigen detection may be useful as an adjunct to microscopic diagnosis in detecting parasites and to distinguish between pathogenic and nonpathogenic infections.  Recent studies indicate improved sensitivity and specificity of fecal antigen assays with the use of monoclonal antibodies which can distinguish between E. histolytica and E. dispar infections.  At least one commercial kit is available which detects only pathogenic E. histolytica infection in stool; several kits are available which detect E. histolytica antigens in stool but do not exclude E. dispar infections.

References:

1. Haque R, Ali IKM, Akther S, Petri WA Jr. Comparison of PCR, isoenzyme analysis, and antigen detection for diagnosis of Entamoeba histolytica infection. J Clin Microbiol 1998;36:449-452.
2. Healy, GR. Serology. In: Ravdin JI, ed. Amebiasis: human infection by Entamoeba histolytica. New York: John Wiley & Sons; 1988. p. 650-719.