Abstract.We assessed an Echinococcus granulosus hydatid fluid antigen-ELISA (EgHF-ELISA) as a serologic prescreening test for E. granulosus infections, supplemented by more specific confirmatory tests, including arc-5 immunoprecipitation and antigen B subunit 8-kD immunoblotting. The diagnostic sensitivity of the EgHF-ELISA was 91%. With regard to the test specificity of the EgHF-ELISA (overall ϭ 82%), we observed relatively frequent crossreactions in tumor patients (6%) and in patients with other parasitic diseases. Cestode-related cross-reactivity can be resolved by the complementary use of E. multilocularis-specific antigens or Taenia solium cysticercosis-specific immunoblotting. Immunoblotting based upon the detection of antibody reactivity to the 8-kD antigen of EgHF, or if appropriately detectable, to the 29-kD and 34-kD bands exhibited a 91% diagnostic sensitivity and an overall specificity of 97% or 94%, respectively. Thus, immunoblotting provided a 99% discrimination between seropositive preoperative cystic hydatid disease cases and cross-reactive non-cestode parasitic infections or malignancies.For primary serologic diagnosis and for support of clinical diagnosis of cystic echinococcosis (cystic hydatid disease [CHD]), the selection of a particular immunodiagnostic test involves consideration of the diagnostic operating characteristics of the technique and the purpose for which it will be used. The diagnostic sensitivity and specificity of the tests vary according to the nature and quality of the antigen and the methodologic sensitivity of the selected technology. A concise definition of the sera used for the assessment of test parameters is essential, with special attention paid to the definition of pre-or post-operative situations respective to CHD serum sampling time point. One of the most specific conventional immunodiagnostic approaches for CHD relies upon the demonstration of serum antibodies precipitating an antigen called antigen 5 by immunoelectrophoresis or similar techniques.1 Diagnostic sensitivities with respect to hepatic CHD have been reported to vary between 50% and 80%. Antibodies to antigen 5 also occurred in serum of human patients with neurocysticercosis 3 and alveolar echinococcosis (AE). 4 Comparative studies showed that 58% of Swiss patients with AE were arc 5-positive compared with 74% of patients with CHD. 5 Resolution of E. granulosus hydatid cyst fluid by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by immunoblotting, resulted in the identification of the arc-5 subunits, including two subunits with relative molecular masses estimated by different laboratories to be between 37 and 38 kD and 20-22-24 kD, respectively. 2 Diagnostic assessment of these two antigens by immunoblotting performed in different laboratories have resulted in the publication of discrepant sensitivity and specificity parameters. 2,6,7 The second major parasite antigen in hydatid cyst fluid is a thermostable lipoprotein called antigen B. The major components of antigen B r...