CONSTRUCTION OF AN INTRINSIC CUTOFF VALUE FOR THE SEROEPIDEMIOLOGIC STUDY OF TRYPANOSOMA-EVANSI INFECTIONS IN A CANINE POPULATION IN BRAZIL- A NEW APPROACH TOWARDS AN UNBIASED ESTIMATION OF PREVALENCE

In the serodiagnosis of tropical infectious diseases, cut-off values a re often established by using sera from individuals living under moder ate climatic conditions, not exposed to the risk of infection (non-end emic controls). This approach guarantees the disease-free status of th e individuals within that control group but leads to an assembly of sa mples which are not representative for the disease-free individuals of the target population (selection bias). Using data from an epidemiolo gical study of Trypanosoma evansi infection in dogs, two alternative m ethods to construct cut-off values for a T. evansi antibody ELISA are described which are solely based on a distribution analysis of the dat a from the endemic animals. By cluster analysis these data could be di vided into 'high', 'intermediate' and 'low responders'. High responder s could also be identified by using the computer-assisted analysis of mixtures (C.A.MAN). Conventional cut-offs were calculated from a group of non-endemic individuals. A receiver operating characteristic (ROC) analysis was performed to demonstrate the impact of the choice of cut -offs on the test specificity and on the estimated seroprevalence amon g the endemic population. The data indicate that distribution analysis , especially the mixture analysis (C.A.MAN), are valuable tools for th e unbiased estimation of seroprevalence when representative negative c ontrols are not available.