The following is a simplified explanation of how titer is determined for antibodies.
A blood serum sample containing antibody is diluted serially (1:2, 1:4, 1:8, 1:16... and so on). Using an appropriate detection method (colorimetric, chromatographic etc.), each dilution is tested for the presence of detectable levels of antibody. The assigned titer value is indicative of the last dilution in which the antibody was detected. For example, if antibody was detected in each of the tubes listed above, but not in a 1:32 dilution, the titer is said to be 16. If it is detected in the 1:2 and 1:4 dilutions, but no others, the titer is said to be 4. Therefore the titer is the degree to which the antibody-serum solution can be diluted and still contain detectable amounts of antibody.