Titer

A titer (or titre) is a way of expressing 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 (i.e, 2−8). A specific example is a viral titer, which is the lowest concentration of virus that still infects cells. To determine the titer, several dilutions are prepared, such as 10−1, 10−2, 10−3,...,10−8.

There are two main kinds of titer testing that one can do. First there is the physical titer; this titer gives one the concentration of virus particles per unit of measurement. The second way to measure viral titers is to perform an infectious titer level. This test tells one the concentration of infectious particles that have the ability to cause infection. A physical titer is much easier and faster to perform but does not always tell one if that level is an infectious amount or not.

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.

The titer of a fat is the temperature, in degrees Celsius, at which it solidifies. The higher the titer, the harder the fat. This titer is used in determining whether an animal fat is considered tallow (titer higher than 40 °C) or a grease (titer below 40 °C).

A titer (when referring to a library titration) is the number of plaque forming units per milliliter. The reason why the titer is important is because we need to make sure that the library has enough viable phage particles to represent all of the original genome. Also, a certain phage density needs to be plated in order to be able to screen the library successfully.