Add PFU (Plaque forming units)

[gschadow]

Plaque forming units, an entitic number, like CFU. This is not quite an arbitrary unit.

WHO EXPERT COMMITTEE ON BIOLOGICAL STANDARDIZATION (55th Edition) WHO Technical Report #932; copyright 2005 ....

5. Virus titration for infectivity

Each working seed lot should be assayed for infectivity in a sensitive assay in cell cultures.

• A plaque assay may be used in Vero or other sensitive cells. Titre should be determined by counting the number of visible plaques developed, and results recorded as PFU/ml.
• An immunofocus assay may also be used to measure virus titre. The assay is based on the visualization of infected areas of a cell monolayer by probing with dengue serotype-specific monoclonal antibodies. Results should be recorded as FFU/ml.
• A tissue culture infectious dose assay may also be used to determine virus titre. Results should be recorded as CCID50/ml.

If international reference standards become available, immunizing doses should be expressed as relative potencies rather than CCID50 because experience shows that this reduces variability between assays.

[gschadow]

More explanation:

Viral infectivity in a sensitive assay in cell culture where the titer is determined by counting the number of visible plaques developed following viral infection of a sensitive cell culture and results recorded as PFU/ml.