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Chronic phase

The chronic phase begins 2 months after the initial infection when the clinical manifestations (if any) of the acute phase disappear, and parasitemia falls to undetectable levels. In most cases, the chronic phase presents as an indeterminate form, which may evolve to the cardiac, digestive, or cardiodigestive forms after years or decades. The diagnosis is made by serological tests, such as indirect hemagglutination, indirect immunofluorescence, and ELISA, all of which have high sensitivity (around 98%) and acceptable specificity. The historical complement fixation reaction (Guerreiro—Machado) is no longer used because of its complexity and because it is no more sensitive or specific than the other tests. According to WHO recommendation,14 diagnosis should be based on the positivity of at least two of the tests mentioned above.

Demonstration of the parasite in blood may be performed by xenodiagnosis, with the classic (4 boxes with 10 triatomines in each) or the artificial method; the latter has several advantages (is more comfortable for the patients, avoids skin reactions to the triatomine bites, and has equal or superior sensitivity than the classical xenodiagnosis). Another method for parasitological diagnosis is hemocul- ture, which shows positivity in about 50% of cases. Positivity of these techniques increases when the examination is performed two or more times.

The PCR method is useful for diagnosis of the chronic phase, mainly in those cases with dubious results on serology.

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