On the Calculation of TCID 50 for Quantitation of Virus Infectivity
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LETTER
On the Calculation of TCID50 for Quantitation of Virus Infectivity Chengfeng Lei1 • Jian Yang1,2 • Jia Hu1 • Xiulian Sun1 Received: 10 March 2020 / Accepted: 15 April 2020 Ó Wuhan Institute of Virology, CAS 2020
Dear Editor 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–Ka¨rber method (Spearman 1908; Ka¨rber 1931), the Reed–Muench method (Reed and Muench 1938), the improved Ka¨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.
that was infected at a 10-7 dilution of virus would also have been infected at a 10-6 dilution. Therefore, at 10-6 dilution of virus there would be 4 cumulative infected units (1 at 10-7 dilution and 3 at 10-6 dilution). Similarly, at 10-5 dilution of virus, the cumulative number of infected units would be 1 (at 10-7) ? 3 (at 10-6) ? 5 (at 10-5) = 9. The number of cumulative non-infected units was calculated in similar fashion based on the assumption that test units were not infected by a given dilution of virus would also be uninfected by a higher dilution of the virus (Burleson et al. 1992). The infection rate was calculated as: Infection rate ¼ number of cumulative positive units : number of cumulative positive units þ number of cumulative negative units
ð1Þ The dilution corresponding to the 50% endpoint (ID50) lay somewhere between the 10-6 (66.7% positive) and 10-7 (14.3% positive) dilutions. The proportionate distance (PD) between these two dilutions is calculated in the following manner: PD ¼
Reed–Muench Method Table 1 shows typical data for titration of virus stocks available in the literature. To calculate the infection rate for case 1, the number of cumulative infected units (‘positive’) was calculated based on the assumption that the test unit Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12250-020-00230-5) contains supplementary material, which is available to authorized users. & Xiulian Sun [email protected] 1
Wuhan Institute of Virology, Center for Biosafety MegaScience, Chinese Academy of Sciences, Wuhan 430071, China
2
University of Chinese Ac
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