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Clin. Vaccine Immunol. doi:10.1128/CVI.00278-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Evaluation of an Indirect Immunofluorescence Assay for Strongyloidiasis as a Tool for Diagnosis and Follow-up

Centre for Tropical Diseases, Sacro Cuore Hospital, Negrar, Verona, Italy, and Medicine and Public Health Department, University of Verona, Italy

^* To whom correspondence should be addressed. Email: zeno.bisoffi{at}sacrocuore.it.

	Abstract

The diagnostic accuracy of an Indirect Immunofluorescence Antibody Test (IFAT) for Strongyloides stercoralis at different serum titers was evaluated. To assess diagnostic sensitivity, sera from 156 patients with known strongyloidiasis were collected. Negative control sera were obtained from a composite group of 427 subjects (blood donors and hospitalized patients). With an area under the Receiver-Operating Characteristic (ROC) plot of 0.98, IFAT showed a high diagnostic accuracy for strongyloidiasis. An antibody titer of = 1:20, with 97% sensitivity and 98% specificity, was identified as the diagnostic threshold with the best overall performance. Cross-reactions were evaluated in 41 additional samples from patients with other known helminth infections, detecting a low-titer IFAT positivity in only one subject with filariasis. A positive IFAT at a dilution of 1:80, was virtually 100% specific, with 71% sensitivity. To test the usefulness of IFAT as a monitoring tool, the changes in specific-antibody titers after treatment in a group of 155 patients were evaluated. A seroreversion or a decrease by two or more titers were observed in 60% of the patients. Response to treatment was directly correlated to the initial antibody titer and a baseline titer of 1:80 was identified as the best predictor of response. In conclusion, a positive IFAT at a dilution of 1:20 is the optimal cutoff for screening. A titer of 1:80, with virtually no false-positive result, is a reliable cutoff for a serological assessment of treatment efficacy and for inclusion in clinical trials.