The hemagglutination assay (HA) is a tool used to screen cell culture supernatant or amniotic-allantoic fluid harvested from embryonated chicken eggs for hemagglutinating agents, such as type A influenza. The HA is not an identification assay, as other agents also have hemagglutinating properties. Live and inactivated viruses are detected by the HA test. Amplification by virus isolation in embryonated chicken eggs or cell culture is typically required before HA activity can be detected from a clinical sample. The test is, to some extent, quantitative as one hemagglutinating unit (HAU) is equal to approximately 5-6 log10 of virus. It is inexpensive and relatively simple to conduct. Several factors (e.g., the quality of chicken erythrocytes, laboratory temperature, laboratory equipment, technical expertise of the user) may contribute to slight differences in the interpretation of the test each time it is run. This chapter will describe the methods validated and used by the US National Veterinary Services Laboratories for screening and identification of hemagglutinating viruses.
Keywords: Avian influenza virus; Hemagglutination; Influenza detection; Type A influenza; Virus titer.