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Short answer
Viral titer (also spelled virus titer, or “titre of virus”) is the concentration of virus in a sample. The number only means something together with its unit and the assay that produced it: PFU, TCID50 and FFU count infectious units, while genome copies from qPCR or ddPCR count nucleic acid whether or not the particle can infect a cell.
Definition and scope
A titer expresses how much virus a preparation contains per unit volume. “Virus titer” and “titre of virus” are the same quantity in US and UK spelling. What differs is what is being counted. An infectious titer counts virus that can infect cells in culture and produce a readable effect. A genome-copy count measures viral nucleic acid in the sample. Two preparations of the same virus can give different numbers by different assays, and by the same assay in different cell lines, so a titer is a measurement of a preparation under stated conditions, not a fixed property of the virus.
This guide is the overview. For the step-by-step plaque method, see the plaque assay guide; for the dilution arithmetic that every endpoint assay depends on, see serial dilution technique.
How virus titer is measured in practice
Plaque assay (PFU)
Infected cell monolayers are scored for plaques, each ideally arising from one infectious unit, giving plaque-forming units (PFU) per mL. A trade review in Lab Manager describes the plaque assay as robust but time-consuming and labour-intensive, and notes that some virus types do not form plaques in culture.
TCID50
The 50% tissue culture infectious dose is the amount of virus that produces cytopathic effect in half of the inoculated cultures. In a 2020 adenovirus study, TCID50 titers were calculated with the Reed and Muench formula (Reed and Muench, 1938), which interpolates the proportionate distance to the 50% endpoint (Sayedahmed and Mittal, Can J Vet Res 2020). The same authors describe TCID50 as one of the preferred tests for adenoviruses that do not easily form plaques. Lab Manager calls the method widely accepted but time-consuming and imprecise.
Focus-forming units (FFU)
Foci assays count infected cells or clusters by staining for a viral antigen. In a 2020 hepatitis C virus study, one FFU was defined as one or more NS5A-positive cells separated from other NS5A-positive cells by at least two NS5A-negative cells (Singh et al., J Virol 2020). Lab Manager notes that foci counting has traditionally been manual and subjective.
Genome copies (qPCR, ddPCR)
Quantitative PCR measures viral genomes. The Singh study used qRT-PCR to quantify genome copies in virus supernatants and reported infectious titer (TCID50) alongside genome copies. Lab Manager lists qPCR as fast and sensitive but affected by primer design, inhibitors and standard-curve quality, and describes droplet digital PCR as counting partitioned droplets without a standard curve.
Other methods
Lab Manager adds that hemagglutination is simple and inexpensive but weak at low titers, ELISA needs virus-specific reagents, transmission electron microscopy is informative but slow, and flow virometry on conventional cytometers is limited to particles of roughly 300 nm and above.
Which count answers which question
| Assay and unit | What it counts | Known failure mode |
|---|---|---|
| Plaque assay (PFU/mL) | Infectious units that form plaques | Some viruses do not form plaques; in one adenovirus study the PFU titer of porcine adenovirus 3 was significantly lower than its TCID50 and FFU titers, which the authors attributed to inefficient plaque formation in PK-15 cells |
| TCID50 | Dose giving cytopathic effect in 50% of cultures | Described as time-consuming and imprecise |
| Focus assay (FFU) | Antigen-positive infected cells or foci under a stated definition | Manual counting can be subjective; the focus definition must be stated |
| qPCR or ddPCR (genome copies) | Viral nucleic acid, infectious or not | qPCR depends on primers, inhibitors and standard curve; impurities such as cellular debris and free nucleic acids complicate counts |
In that adenovirus comparison, the authors suggested that focus-forming units could give more consistent results than PFU or TCID50 for adenoviruses and adenoviral vectors. That is a suggestion about that virus family and cell line, not a rule that FFU is the best assay.
Where it matters for labs and clinics
Titer is the number used to set the amount of virus added to cells. Lentiviral work is a common example: the titering step in a lentiviral transduction protocol determines how a vector stock is dosed, and the choice of vector (see viral vector types) affects which assays suit it. For clinical gene and cell therapy products, FDA’s final guidance on potency tests (January 2011) states that potency measurements are designed specifically for a particular product and does not recommend specific types of potency assays. Which assay supports a given product is therefore a product-specific decision.
Reporting checklist
- State the unit (PFU/mL, TCID50/mL, FFU/mL, genome copies/mL), not just a number.
- Name the assay, including the focus definition for FFU or the calculation method for TCID50.
- Name the cell line used for readout; the same virus can read differently by cell line.
- When reporting genome copies, report them as genome copies, and where available give the infectious titer next to them rather than substituting one for the other.
Common misunderstandings
Genome copies are not infectious particles
qPCR counts nucleic acid, which includes material from particles that cannot infect and free nucleic acids in the sample. The two numbers answer different questions, which is why some studies report them side by side.
PFU and TCID50 are not interchangeable
They are different endpoints. Do not convert between them with a fixed factor unless a source for your virus and cell system states one.
One assay does not fit every virus
Viruses that do not form plaques need an endpoint or focus method instead; the assay has to match the virus and cell system.
Frequently asked questions
What is viral titer?
The concentration of virus in a sample, expressed in a stated unit such as PFU/mL, TCID50/mL, FFU/mL or genome copies/mL.
Is virus titer the same as titre of virus?
Yes. Titre is the UK spelling; the quantity is the same.
Which assay should I use?
It depends on the virus and cell system. If the virus forms plaques in the chosen cell line, a plaque assay is one option; if it does not, the adenovirus study cited below describes TCID50 as a preferred test and suggests focus-forming assays as an alternative.
Sources
- Sayedahmed EE, Mittal SK. A potential approach for assessing the quality of human and nonhuman adenoviral vector preparations. Can J Vet Res 2020;84(4):314-318. https://pmc.ncbi.nlm.nih.gov/articles/PMC7491003/ (checked 2026-10-29)
- Singh D et al. J Virol 2020;94(3):e01327-19. https://pmc.ncbi.nlm.nih.gov/articles/PMC7000968/ (checked 2026-10-29)
- Lab Manager. Viral Titer Quantification: Balancing Traditional and Modern Approaches for Comprehensive Analysis (trade article). https://www.labmanager.com/viral-titer-quantification-balancing-traditional-and-modern-approaches-for-comprehensive-analysis-31559 (checked 2026-10-29)
- US FDA. Potency Tests for Cellular and Gene Therapy Products, final guidance, January 2011. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/potency-tests-cellular-and-gene-therapy-products (checked 2026-10-29)
