|This article does not cite any references or sources. (January 2009)|
An antibody titer is a measurement of how much antibody an organism has produced that recognizes a particular epitope, expressed as the inverse of the greatest dilution that still gives a positive result. ELISA is a common means of determining antibody titers.
For example, the indirect Coombs test detects the presence of anti-Rh antibodies in a pregnant woman's blood serum. A patient might be reported to have an "indirect Coombs titer" of 16. This means that the patient's serum gives a positive indirect Coombs test at any dilution down to 1/16 (1 part serum to 15 parts diluent). At greater dilutions the indirect Coombs test is negative. If a few weeks later the same patient had an indirect Coombs titer of 32 (1/32 dilution which is 1 part serum to 31 parts diluent), this would mean that she was making more anti-Rh antibody, since it took a greater dilution to abolish the positive test.
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.