The commercial ELISA kit (IDEXX HerdChecky PRRS) has been the main serodiagnostic tool for PRRS virus (PRRSV) in North America and many pig producing regions throughout the world. Although the kit has been mostly a reliable test for detecting PRRSV exposure at the herd level, the specificity of the kit has been frequently challenged by the occurrence of unexpected false positive results at varying rates. An indirect fluorescent antibody (IFA) test has been commonly employed to confirm or disprove the suspect false positive reactivity in the ELISA. However, the reliability of test results have been often questioned due to the subjectivity in determination of test result and perceived lower sensitivity. The current study was conducted first to develop immunoassays (Western immunoblot and microsphere immunoassay) based on recombinant N protein of PRRSV as well as bELISA using the existing commercial ELISA kit (2XR). Then diagnostic performances of four different serologic assays (Western immunoblot, microsphere immunoassay, bELISA and new IDEXX's PRRS ELISA kit X3) was evaluated on the same set of experimental and field serum samples in comparison to ELISA 2XR kit to establish a confirmatory testing algorithm when suspect false positive (SFP) results occur in ELISA 2XR. All assays showed excellent performance on experimental samples, meaning that all tests are valid for specific and sensitive detection of the target antibody. Diagnostic sensitivity and specificity of the assays related to ELISA 2XR, however, varied on the field samples, although all of them greatly reduced the number of SFP samples. By combining two different assays (particularly ELISA X3 and Western immunoblotting), the SFP results could be reduced by as high as 73.5% which provide higher confidence in determining the PRRS status of animal when SFP results occur.