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Laboratory Diagnosis

There is a complement fixation test (Machado-Guerreiro reaction) which becomes positive in the acute stage at one month post-infection and remains positive thereafter. The Machado-Guerreiro reaction may present a high incidence of false positives and negatives, resulting in a low sensitivity and specificity test.

There is a precipitin test (hemoagglutination reaction) which is 95% positive in the early stages. The immunofluorescence and ELISA tests are highly sensitive and specific, although false-positive reactions with malaria, leprosy and leishmaniasis can occur. The xeno test is diagnostic: uninfected laboratory-raised bugs are fed on a patient, then examined 30 days later for metacyclic trypanosomes in their hindgut or feces. Xenodiagnosis can be used to check serology, isolate strains, and evaluate medical (chemo) therapy.

Direct microscopic search for T. cruzi in peripheral blood smears may be attempted only in the first few weeks of the acute stage (Fig. 4.4A). It is rare to find the parasites in the blood after the sixth week. They are seldom identified during the chronic stage. In the acute stage, there is a leukocytosis with marked lymphocytosis or monocytosis, a moderate eosinophilia, and an elevated sedimentation rate.

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