Tests The HI test is frequently used for detection and quantitation of humoral antibody for PPV. Antibody sometimes can be detected as early as 5 days after swine are exposed to live virus, and it may persist for years. Sera examined by the HI test are usually pretreated by heat inactivation (56?C, 30 minutes) and by adsorption with erythrocytes (to remove naturally occurring hemagglutinins) and kaolin (to remove or reduce nonantibody inhibitors of HA). Trypsin also has been used to remove nonantibody inhibitors of HA. Parameters of the HI test have been studied in detail.