A titer (BE: 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 or saline in repeated steps. 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.
Titers are expressed in their highest positive dilution, e.g. 1:1, 1:2, 1:4, 1:8 etc. where the second number is always 2x. Sometimes, the sample is diluted from the outset, leading to different base numbers but a similar exponential increment (e.g. 1:20, 1:40, 1:80, 1:160, 1:320).
See also