HIV and Herpes both form syncytia Direct particle count . Mix virus with known number of latex particles, spray into EM grid, and count. Dulbecco p. 879. Hemagglutination assay . The most common indirect method of measuring the number of virus particles is the hemagglutination assay.
Many animal viruses adsorb to the red blood cells of various animal species.
In practice, the maximum number of cells with which any particular virus particle can combine is two, because red cells are far bigger than viruses. Pattern method . Since the number of virus particles necessary for this is readily calculated, hemagglutination serves as a highly accurate and rapid method of quantitating virus particles. It was and still is particularly useful in studies with myxoviruses, particularly influenza virus, and many others. Non-agglutinated cells roll to bottom, form button.