Table 3.

Factors influencing infectivity assays for viruses.

Primary vs. continuous cell lines	Generally primary cell lines are more sensitive
Specific cell line	Numbers of viruses detected and types of viruses are dependent on the specific cell line; how many passages of the cell line have been performed
Assay conditions	Rocking cells, addition of certain enzymes and polyelectrolytes (e.g., trypsin), PFU vs TCID50; monolayer vs. suspended cells.
Ratio of viral particles to infectivity	Much lower for viruses directly isolated from human feces vs. laboratory grown viruses
Number of passages of the sample in cell culture	Passage of negative samples on first passage onto a second passage results in greater numbers of virus detected in a sample

PFU = plaque forming units; TCID_50 = Tissue culture infectious dose.