Titer

Overview
A titer (or titre) is the unit in which the analytical detection of many substances is expressed. It is the result of a titration. Generally, the test is performed on an undiluted sample, and then repeated when the sample is mixed with 100% water, saline, or other diluent in repeated steps (a serial dilution). If the test is still positive, then high titers of the detected substance are said to be present.

Many tests are positive when performed on an undiluted sample, but rapidly become negative after repeated dilution. These tests may only be of real significance if the titer is high, while lacking this significance when performed on the raw sample. A control substance may be tested alongside the sample, and/or statistical methods are used to distinguish positive from negative results.

Some samples may actually give stronger reactions as they are diluted with a diluent. As further dilutions are made the reactions become less pronounced and eventually cease as the original sample is diluted to the point of no reaction. This is referred to as a prozone phenomenon.

Titers are expressed in their highest positive dilution, e.g. 1:1, 1:2, 1:4, 1:8 or 1:1, 1:10, 1:100, 1:1000 where the second number is always a power of the dilution factor (e.g. 2x, 10x). Sometimes, the sample is diluted from the outset, leading to different multiplers with a similar exponential increment (e.g. 1:20, 1:40, 1:80, 1:160, 1:320).