Two major groups of hantaviruses are recognized based on clinical presentation. The first group includes Sin Nombre virus, which causes hantavirus pulmonary syndrome, a severe and possibly fatal form of acute respiratory distress. A second group of hantaviruses (including Seoul, Hantaan, Dobrava, and Puumala viruses) causes hemorrhagic fever with renal syndrome, a condition not typically seen in the United States. Sera are initially screened for IgG and IgM Antibody recognizing the nucleocapsid protein common to all hantaviruses. All equivocal and positive samples are further tested in confirmatory assays to detect antibody specific for Sin Nombre virus. A positive result in the screening ELISA but a negative result in a confirmatory assay may indicate reactivity to a hantavirus causing hemorrhagic fever with renal syndrome.