Hemagglutination Inhibition Assay — HI / HAI Test

The Hemagglutination Inhibition (HI) Assay is a classical serological test used to detect and quantify antibodies against hemagglutinating viruses — most notably influenza and Newcastle disease virus — in animal and human serum. Widely employed in veterinary diagnostics, vaccine efficacy evaluation, and epidemiological surveillance, it relies on the principle that specific antibodies in serum will block a known quantity of virus from agglutinating red blood cells.