TCID50mL

TCID50 mL (tissue culture infectious dose 50 per milliliter) is a quantitative virological assay used to estimate the concentration of infectious virus particles in a liquid sample. The value represents the dilution of a viral suspension that will produce a cytopathic effect (CPE) in 50 % of inoculated cell culture wells, expressed as the number of infectious units per milliliter.

The assay is performed by preparing a series of serial dilutions of the viral sample, typically ten‑fold, and adding each dilution to multiple replicate wells containing a susceptible cell line. After an incubation period sufficient for viral replication, wells are scored for presence or absence of CPE. The proportion of positive wells at each dilution is used to calculate the 50 % endpoint, most commonly by the Reed–Muench or Spearman–Kärber methods. The resulting TCID50 mL value is the reciprocal of the dilution that yields 50 % CPE, multiplied by the inverse of the inoculum volume.

TCID50 mL is widely employed in vaccine development, antiviral drug testing, and quality control of viral stocks. It provides a functional measure of infectivity, complementing molecular techniques such as quantitative PCR that detect viral genomes but do not assess viability. The metric is also useful for comparing the potency of different viral preparations or for standardising challenge doses in animal studies.

Limitations of the assay include variability due to cell line susceptibility, observer bias in CPE scoring, and reduced sensitivity for viruses that cause subtle or delayed cytopathic changes. Results are reported with confidence intervals to reflect statistical uncertainty. Related concepts include plaque‑forming units (PFU mL) and median lethal dose (LD50), which measure viral infectivity and pathogenicity in different experimental contexts.