titer

A titer is a measure of the amount or activity of a substance in a solution, typically determined by sequential dilutions and an assay. It is usually expressed as the reciprocal of the highest dilution that still yields a detectable result. For example, a titer of 1:128 indicates that a 128-fold dilution remains positive, whereas a higher titer implies a greater amount or activity of the substance being measured.

In serology, antibody titer describes the concentration of antibodies in a person’s serum. It is determined by serial dilutions in tests such as ELISA, agglutination, or virus neutralization assays. The endpoint titer is the last dilution that meets the positive criterion. Higher titers indicate more antibodies, but clinical interpretation depends on the specific assay, pathogen, and established cutoff values.

In microbiology and virology, titers quantify infectious agents or their activity as PFU/mL (plaque-forming units per milliliter) or TCID50/mL, or describe antibody neutralizing titers. The underlying principle is the same: perform serial dilutions and observe a readout to identify the limit of detectable activity. Different units reflect the assay type, not a universal concentration.

Limitations include assay variability, lack of universal standards, and dependence on laboratory method and conditions. Standardization initiatives and reporting practices, such as using international reference standards and geometric mean titers, improve comparability. Titers are used to assess immune status, guide vaccination decisions, evaluate vaccine responses, and screen convalescent plasma, but they do not provide an absolute measure of immunity.