Serum samples belonging to three outbreaks in Argentina (47 patients) taken at different times post-ingestion were analysed employing IIF and ELISA simultaneously. Results show that: a) the number of patients diagnosed by a unique technique, especially by ELISA (31 patients), was lower than the one obtained by the simultaneous use of both assays (38 patients); b) four patients out of the seven diagnosed by a unique technique were negative by the other assay over the period of time evaluated. Therefore, it can be concluded that the use of a sole immunoserological technique can not only lead to the delay in the detection but also to the misdiagnosis of this parasitic infection.