infections
Diagnosis of Infectious Disease
Antibody tests
Antibody tests are usually done on a sample of the infected person’s blood because antibodies circulate in the blood. They also can be done on samples of cerebrospinal fluid or other body fluids. Antibodies are substances produced by a person's immune system to help defend against infection. They are produced by certain types of white blood cell when these white blood cells encounter a foreign substance or cell. It typically takes several days to produce the antibody. An antibody recognizes and targets the specific foreign substance ( Overview of the Immune System ) that triggered its production, so each antibody is unique, made for a specific type (species) of microorganism. If a person has antibodies to a particular microorganism, it means that the person has been exposed to that microorganism and has produced an immune response. However, because many antibodies remain in the bloodstream long after an infection has resolved, finding antibodies to a microorganism does not necessarily mean the person is still infected. The antibodies may remain from a previous infection. Finding antibodies to a microorganism in a person's blood does not necessarily mean that the person is still infected because the antibodies may remain from a previous infection. Doctors may test for several antibodies, depending on which infections they think are likely. Sometimes doctors just test whether an antibody is present or not. But usually they try to determine how much antibody is present. They determine the amount of antibody by repeatedly diluting the sample in half until it no longer tests positive for the antibody. The more dilutions it takes until the test is negative, the more antibody there was in the infected person's sample. Because it takes several days to weeks for the immune system to produce enough antibody to be detected, diagnosis of an infection may be delayed. Antibody tests done right after people become ill are often negative. Thus, doctors may take one sample immediately and then take another one several weeks later to see whether antibody levels have increased. If levels of an antibody are low on the first test after people become ill, finding an increase in the antibody levels several weeks later suggests an active, current or recent (rather than a previous) infection. 