A titer (or titre) is a measure of concentration. Titer testing employs serial dilution to obtain approximate quantitative information from an analytical procedure that inherently only evaluates as positive or negative. The titer corresponds to the highest dilution factor that still yields a positive reading; for example, positive readings in the first 8 serial twofold dilutions translate into a titer of 1:256. A specific example is viral titer. To determine the titer, several dilutions will be prepared, such as 10^-1, 10^-2, 10^-3,...,10^-8. The lowest concentration of virus that still infects cells is the viral titer.

Many traditional serological tests such as hemagglutination or complement fixation employ this principle. Such tests can typically be read visually, which makes them fast and cost-effective in a 'low-tech' environment. The interpretation of serological titers is guided by reference values that are specific for the antigen or antibody in question; a titer of 1:32 may be below the cut-off for one test but above for another.

See also

• Serology
• Titration