On the Calculation of TCID50 for Quantitation of Virus Infectivity

The most important property of a virus is its infectivity. To measure infectivity, one can assay viral replication in cells to obtain a titer for a given virus stock. A titer is defined as a given number of infectious viral units per unit volume, and an infectious unit is the smallest amount of virus that produces recognizable effects [e.g., cytopathic effect (CPE), dot blot immunoreactivity]. The median tissue culture infectious dose (TCID₅₀) is defined as the dilution of a virus required to infect 50% of a given cell culture. Several methods have been developed to calculate the TCID₅₀ including the Spearman–K?rber method (Spearman 1908; K?rber 1931), the Reed–Muench method (Reed and Muench 1938), the improved K?rber method (Sun 1963), the Weil method (Meynell GG and Meynell E 1970), and probit/logit regression models (Finney 1971). In this letter, we analyze datasets using three popular methods for calculating the TCID₅₀ in Excel based calculator (Supplementary file 1). We also compared these results with those of probit/logit regression models and discuss the differences among these methods.