One way around this is to use a functional definition of infectious particles rather than a strict count, such as a plaque forming unit.

A titer (when referring to a library titration) is the number of plaque forming units per milliliter.

The concentration of plaque forming units can be estimated by the number of plaques (regions of infected cells) formed after a few days.

Titers of plaques can be found by diluting the samples and counting plaque forming units (PFUs).

Depending on the virus, the plaque forming units are measured by microscopic observation, fluorescent antibodies or specific dyes that react with infected cells.

Viral plaque assays determine the number of plaque forming units (pfu) in a virus sample, which is one measure of virus quantity.

The cDNA library comprised 7.0x10 5 plaque forming units and was screened without amplification using the human SRY probe.

Also included were postimmunization serial sera from eight normal healthy women who had received 6000 plaque forming units of the live attenuated Towne vaccine as a single subcutaneous injection.

After elution, the phage can be plated and DNA extracted from individual plaque forming units, restricted and the appropriately sized fragments extracted after separation by PAGE.

Plaques are generally counted manually and the results, in combination with the dilution factor used to prepare the plate, are used to calculate the number of plaque forming units per sample unit volume (pfu/mL).