The TPHA (Treponema Pallidum Hemagglutination Assay) is a blood test used to detect antibodies against Treponema pallidum, the bacterium responsible for syphilis. The test works by using red blood cells coated with antigens from Treponema pallidum. If a person has antibodies to the bacteria, they will bind to these antigens and cause the red blood cells to clump together, or hemagglutinate.