A hemagglutination inhibition assay (HAI) is an indirect serological test that detects the presence of virus-specific antibodies in patient serum. In a positive test, these antibodies bind to viral hemagglutinins and prevent them from clumping red blood cells (erythrocytes). In a microtiter plate, this lack of clumping (reaction) allows the red blood cells to settle to the bottom, forming a distinct small red dot in the center of the well.