Hemagglutinin Inhibition Test

The Hemagglutinin Inhibition (HI) Test is a laboratory method used to measure the presence of specific antibodies against influenza viruses in a person's blood. The test works by mixing the blood serum with a virus; if antibodies are present, they will prevent the virus from clumping red blood cells together—a process called hemagglutination. By observing whether the red blood cells remain separate, scientists can determine the level of immunity the person has against the flu. This test helps assess vaccination effectiveness and prior exposure to the virus.