Procedure
The detailed conditions depend on the type of virus or bacteria being assayed since certain pH values and ionic strengths can impact the activity of the proteins of interest in a difficult to predict manner.
Normally, a virus dilution (e.g. 2-fold from 1:4 to 1:4096) will be applied to an RBC dilution (e.g. 0.1% to 0.7% in steps of 0.2%) for approx. 30 min, often at 4C, otherwise viruses with neuraminidase activity will detach the virus from the RBCs. Then the lattice forming parts will be counted and the titer calculated.
Virus concentration in virions per millilitre = 107 x HA titer.
The titer of a hemagglutination assay is determined by the last viable "lattice" structure found. This is because it is at the point where, if diluted anymore, the amount of Virus particles will be less than that of the RBCs and thus not be able to agglutinate them together.
For bacteria, depending on species, a bacterial dilution will be applied to an equal part RBC dilution and then incubated for 30 min to an hour at an optimal growth temperature before being observed.
Read more about this topic: Hemagglutination Assay