In the clinical laboratory, titer is a unit of measurement. It is most often thought of as the lowest dilution of a substance in which a reaction takes place. It is usually expressed as a ratio (i.e., 1:20). For example, serum containing an antibody can be diluted with saline in a serial manner producing dilutions 1:5, 1:10, 1:20, 1:40, etc. If the lowest dilution that a reaction can still be detected between the antibody and the antigen it is directed against is 1:20, then that is the result of the antibody titer.