Titer (or titre) is a quantitative measure of the concentration of a substance, typically defined as the highest dilution of a sample that still produces a detectable reaction in an assay, such as antibody binding or viral infectivity.
Explanation
In immunology and virology, a titer expresses how much of a specific antibody, antigen or virus is present in a sample. Determination involves serially diluting the sample and testing each dilution for a defined endpoint, such as agglutination, neutralisation or cytopathic effect. The titer is reported as the reciprocal of the highest dilution that yields a positive result. For example, an antibody titer of 1:160 indicates that the antibody is still detectable when the serum is diluted 160‑fold. Viral titers measure infectious virus particles and can be expressed as plaque‑forming units per millilitre (PFU/mL) using plaque assays, tissue culture infectious dose (TCID50) or focus‑forming units. This quantitative information guides clinical decisions, vaccine efficacy assessments and laboratory experiments. Titer measurements depend on the sensitivity of the assay, sample quality and defined cutoff values.
Measurement and Applications
Common applications of titers include evaluating immune responses to vaccination by measuring antibody titers against pathogens such as measles, hepatitis B or SARS‑CoV‑2; determining viral loads in research and quality control of vaccine lots; and quantifying haemagglutination or neutralising antibodies for influenza surveillance. In blood banking, titers of maternal antibodies can predict the severity of haemolytic disease of the foetus and newborn. Microbiologists estimate bacteriophage titers using plaque assays, while virologists determine TCID50 to compare infectivity of viral stocks. Accurate titer determination requires proper serial dilution techniques, replicates and controls.
Titers provide a standardised way to quantify biological molecules and assess functional activity. Understanding and interpreting titer values are essential for diagnosing infections, monitoring immunity and conducting laboratory research.
Related Terms: Dilution, Antibody, Serology, Plaque assay, Endpoint