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Serological tests that identify circulating ameba-specific antibodies are extremely valuable. The circulating antibodies were initially described in the early part of this century, through complement fixation and precipitation. Later, indirect hemagglutination and indirect fluorescent antibody tests (which are accurate and positive at low titers when tissue invasion occurs), as well as latex agglutination and other technologic advances heralded the progress in laboratory diagnosis of this parasitic disease. Currently, the enzyme-linked immunosorbent assay (ELISA) test for detection of the galactose-inhibitable adherence protein of E. histolytica (necessary for amebic adherence) in serum and feces appears to be the most reliable and sensitive serology test with a virtual absence of false-negative results and a low 3.6% of false-positives. The ELISA and counterimmunoelectrophoresis (CIET) tests are highly sensitive and rapid; the CIET can be completed in 3 hours and the ELISA titers in 1 day. If used together there are virtually no false-positives. The titers of ELISA are lower in intestinal amebiasis than in asymptomatic carriers; both tests are accurate for extraintestinal amebiasis. Such tests may remain positive for up to 2 years after the disease is eradicated, and can therefore be used to establish the diagnosis, but are less reliable as a therapeutic index.

Low serology titers may be unhelpful, requiring confirmation of invasion by microscopy. Also, serological testing is limited in the asymptomatic individual or as a screening procedure where antibody titers are negative or weak. Patients with high titers and positive imaging tests or other clinical evidence of extraintestinal disease should undergo therapy with both a luminal and a tissue amebicide.

Circulating E. histolytica-specific antibodies appear approximately seven days after the initiation of symptoms of invasive amebiasis. Thus, the absence of serum antibodies seven or more days after the onset of symptoms virtually eliminates amebiasis as a diagnostic consideration. A recombinant technology made of E. histolytica proteins (SREHP/MBP) has enhanced the serodiagnostic ability to detect invasive amebiasis with a sensitivity of 79% and a specificity of 87%. Specific monoclonal antibodies against E. histolytica via immunofluoresence permits differentiation between the invasive and noninvasive abilities of the parasite. This technique requires samples of fresh feces. Reliability of this immunofluoresence test of 100% has been recently reported. The genotypes of invasive and noninvasive infestations correlate with the 1:10 ratio of symptomatic versus nonsymptomatic patients.

Immunoblot methods utilizing immunoglobulin A (IgA) antibodies of serum from patients with hepatic and intestinal amebiasis have shown that patients with hepatic abscess reacted to proteins greater that 200 kDa, while those with intestinal amebiasis reacted to proteins in the range of 145 kDa. IgG does not appear to increase in intestinal amebiasis and carriers, while a significant increase takes place in patients with active hepatic disease.

Finally, there is a latex agglutination test that can be performed at home for patients with intestinal invasive amebiasis. This test has an excellent correlation with the serology tests. Culture of E. histolytica can also be achieved with special media, but morphology and serology are more practical for identification of cysts and trophozoites and thus for confirmation of diagnosis.

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