TCID50

TCID50, or tissue culture infectious dose 50%, is a quantitative assay metric used in virology to estimate the amount of infectious virus required to produce a detectable infection in 50% of cell cultures inoculated under defined conditions. It is typically expressed as the dilution of virus that infects 50% of wells, or as TCID50 per milliliter of stock. It differs from plaque-forming units (PFU) by relying on the infection of cells and cytopathic effect rather than counting plaques; it can also be evaluated by non-lytic readouts such as immunostaining for viral antigen.

The assay involves preparing serial dilutions of a virus and inoculating replicates of susceptible cell cultures. After an incubation period, wells are scored as positive or negative for infection (usually by cytopathic effect or antigen expression). The 50% endpoint is estimated using statistical methods such as the Reed–Münch or Spearman–Kärber calculations, yielding a logTCID50 value.

Limitations include dependence on cell type, virus strain, inoculation conditions, and reporter readouts; TCID50 values are not directly comparable between laboratories. Some viruses do not produce clear cytopathic effects, necessitating alternative readouts. Applications include characterization of viral stocks, titration for vaccine preparations, and potency testing. While widely used, TCID50 provides an estimate of infectious titer rather than a direct count of infectious particles, and is one of several methods used to assess viral infectivity.