What Is TCID50 and Why Does It Matter?
TCID50 (Tissue Culture Infectious Dose 50%) is the gold standard metric for quantifying viral infectivity in cell culture. It represents the dilution of a virus stock at which 50% of inoculated cell culture wells show cytopathic effect (CPE).
Unlike plaque assays (which measure PFU), TCID50 assays work for viruses that do not form distinct plaques — including many respiratory viruses, retroviruses, and some enteroviruses. This makes TCID50 the preferred method across virology, vaccinology, and gene therapy.
The TCID50 assay is simple to perform but surprisingly error-prone to calculate. Common errors include incorrect cumulative counting direction, using the wrong interpolation formula, and failing to account for non-standard dilution factors. This calculator eliminates these errors by showing every intermediate step.
How the Methods Work
This calculator implements three established methods for TCID50 estimation:
Reed-Muench (1938) builds a cumulative table of positive and negative observations across all dilutions, then uses linear interpolation to find the exact point where the cumulative positive rate crosses 50%. It is the most widely cited method and works even when the data does not include a complete endpoint (100% to 0% transition).
Spearman-Kärber is a distribution-free estimator that computes the log-mean of the infectivity distribution directly from the proportion of positive wells at each dilution. It requires complete endpoints (at least one dilution at 100% and one at 0%) but does not require interpolation.
Improved Kärber (Lei et al., 2021) extends Spearman-Kärber with a variance estimate, enabling 95% confidence intervals. This is particularly valuable for comparing titers across experiments or for regulatory submissions that require uncertainty quantification.
All three methods are deterministic — the same inputs always produce the same outputs. This is the foundation of reproducibility.
