Add FFU?

[gschadow]

HO 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 using immunofocus technology. The titer is determined by visualizing infected areas of a cell monolayer by probing with virus-specific monoclonal antibodies and results recorded as FFU/ml.

[gschadow]

Unfortunately things are still murky with PFU and FFU, because we didn't make a unit for CFU either. These are just counts of some phenomenon, their IUPAC property is just number or and PFU/mL is a volumic number. The key of the meaning is the assay. Since the assay is determining the meaning of the result completely, we need an obervation code to carry that information safely. The Ingredient (Role) quantity is designed for more clear cut cases, where you have SI units which are understood as mass, volume, and amount of substance.

[gschadow]

CBER requests change:

> FFU: CBER does not agree that we should limit focus forming unit (FFU)
> with the phrase 'immuno' and the definition  provided by Gunther which
> states that monoclonal antibodies are used is too specific, as other
> types of antibodies can be used to probe.  Additionally vital dyes which
> do not work through the antibody/antigen reactions would be excluded
> from this definition.

Action: request text to replace.

[gschadow]

Modification proposed [15636]. Waiting for OK from CBER before closing ticket.

[gschadow]

This has been finalized.