Our promise to you:
Guaranteed product quality, expert customer support.
24x7 CUSTOMER SERVICE
CONTACT US TO ORDER
Virus titer testing is a core quantitative assay for viral seed bank release. It measures infectious virus titer for master and working virus seed banks, ensuring consistent and well‑characterized virus activity for vaccine and gene therapy production.
Per ICH Q5A(R2) and EP 2.6.8, infectious titer determination is mandatory for viral seed bank characterization. Infectious titer reflects the ability of virus particles to infect host cells and replicate, directly linking to batch consistency, MOI calculations, dosing, stability assessment, and process control. We offer three core methods: TCID50 endpoint dilution, plaque‑forming unit (PFU) assay, and qPCR absolute quantification (as a reference).
Infectious titer measures only functional, replication‑competent virus particles, unlike physical titer, which counts total particles, including empty capsids and debris.
TCID50 (50% Tissue Culture Infectious Dose) is the dilution that causes cytopathic effect (CPE) in 50% of inoculated cell cultures. It is suitable for viruses that do not form plaques or infect monolayers, such as AAV. Results are reported as TCID50/mL.
Serial dilutions of the virus are mixed with host cells and overlaid. Each infectious virus particle lyses cells to form a visible plaque. Plaques are counted, and the titer is calculated as PFU/mL. This is a direct, quantitative measure of infectious virus.
qPCR quantifies viral genome copies via a standard curve, reported as GC/mL. It is fast (hours), but cannot distinguish infectious particles from defective particles or free nucleic acids. qPCR results are typically higher than the infectious titer. qPCR is a complementary reference, not a replacement. Viral seed bank release must rely on infectious titer (TCID50/mL or PFU/mL).
| Aspect | TCID50 | PFU Assay | qPCR |
| Target | Infectious virus (50% infection) | Infectious virus (plaque formation) | Genome copies (including defective) |
| Unit | TCID50/mL | PFU/mL | GC/mL |
| Turnaround | 5‑14 days | 3‑14 days | 1‑2 days |
| Throughput | Medium‑high (96‑well) | Low (6/12/24‑well) | High (96/384‑well) |
| Subjectivity | Moderate (visual CPE) to low (qPCR endpoint) | Low | Very low |
| CV | Visual CPE: intra‑assay 45%, inter‑assay 66%; qPCR endpoint: intra‑assay 9%, inter‑assay 73% | 10‑100% | <10% |
| Applicable viruses | Non‑plaque‑forming (AAV, herpesvirus, etc.) | Plaque‑forming (adenovirus, poxvirus, lentivirus) | All types |
For titer testing strategy, method validation, or IND/BLA submission support for your viral seed bank, contact Creative Biogene’s technical team for a customized plan.
Q1: Is virus titer testing mandatory for all viral seed banks?
A: Yes. ICH Q5A(R2) and EP 2.6.8 require infectious titer determination for MVSB and WVSB release. It is essential for MOI calculation, dosing, batch consistency, and stability studies. 9 CFR 113.8 adds specific rules for live virus vaccines.
Q2: Why is TCID50 highly variable, and how can it be optimized?
A: Variability comes from subjective visual CPE reading (intra‑assay CV up to 45%), cell/helper virus batch variation, and operator skill. Optimization: use qPCR or ddPCR as the endpoint (intra‑assay CV down to 9%), use standardized cell and helper virus banks, and use negative-pressure PCR labs to reduce background. A 2012 study showed qPCR endpoint reduced intra‑assay CV from 45% to 9%.
Q3: What are the special requirements for AAV titer testing?
A: AAV does not form plaques; TCID50 is the standard method, using HeLa‑derived cells expressing rep/cap and adenovirus as helper. The endpoint is determined by qPCR detecting AAV genome replication. Challenges include high variability, stability of integrated genes, and safety concerns with the wild-type helper virus. Optimizations: standardized cell/helper banks, qPCR endpoint, ddPCR endpoint (mean %CV 34.9 vs. qPCR 42.5 from 18 runs), and improved primer/probe design.
Q4: Can qPCR replace TCID50 or plaque assay for viral seed bank release?
A: No. qPCR measures genome titer (GC/mL), which includes infectious particles, empty capsids, and free nucleic acids. The infectious‑to‑total particle ratio varies by virus type, purification, and storage conditions. Viral seed bank release must use TCID50 or plaque assay for infectious titer. qPCR may be used as a rapid screening tool for in‑process control or as an endpoint readout for TCID50, but not as a standalone release method.
Copyright © 2026 Creative Biogene. All rights reserved.