Humoral antibodies that have been stimulated by infection can be used in blood tests in the diagnostic laboratory to diagnose what the infection is or to screen a herd for the presence or absence of an infection.

Numerous serological tests are available e.g. agglutination tests, conglutination tests, complement fixation tests, fluorescent antibody tests, ELISAs etc. Different tests are useful for different infections. The laboratory has to decide which one is best in each case.

When carrying out such tests, either the laboratory can use commercially available antiserum to test against organisms that have been isolated, or they can use double serum samples (two samples taken over a period of time) from the sick pigs to test for specific antibodies using known antigens. Why double serum samples? Because if just one sample is taken and it is positive you do not know whether the antibodies are carried over from an old infection, which has long since gone, or whether it is associated with your current disease problem. The first sample is called "Acute" the second called "Convalescent" which is taken 7 to 14 days later. If the antibodies are due to the current infection, they will be rising from zero or very low to high. If they remain level or fall it is probably a past infection. When antibody levels rise it is called a rising titre, a titre being the term used to express the concentration of specific antibody in a given serum sample. This is measured by the amount the serum has to be diluted before the test becomes negative (i.e. the antibodies have been diluted to a non-detectable level). So if the titre rises from, say, 1:10 to 1:100, it means that the laboratory had to dilute the second serum sample by 10 times indicating infection. Most tests have a bottom threshold titre below which the test is deemed negative. They may also have a narrow middle band which is deemed to be suspicious and a higher titre above which the test is deemed positive.