What do the test results mean?
Antibody tests usually involve mixing the patient’s sample with a known antigen, the substance that the antibody is directed against or produced in response to, and seeing if a reaction takes place. If an antibody is present and binds to the known antigen, the formation of the antibody-antigen complex can be measured.
There really isn't a "normal" antibody concentration since people produce antibodies at different rates. Patients with compromised immune systems may not be able to respond normally, producing fewer antibodies and/or responding more slowly to antigen exposure. The meaning of a particular test result depends on the patient’s symptoms and the specific circumstances that led to testing.
Results may be reported in a qualitative manner as "detected" or "not-detected" in the case of antibodies to agents causing chronic infections (such as HIV), where any amount of antibody is considered meaningful. They may be reported out as "greater than" a particular cutoff level if immunity is being checked (above that level - which varies depending on the microorganism involved - a person is usually considered to be protected), or as "immune" or "non-immune" (meaning that the person does or does not have sufficient antibodies to ward off infection). Results may also be reported as a number representing a concentration.
Detection of IgM antibodies tends to indicate a recent initial exposure to an antigen whereas detection of total or IgG antibodies indicates exposure some time ago.
Antibody titers are sometimes used to evaluate how significant a positive antibody level is. These titers involve diluting the sample – creating and testing serial (increasing) dilutions. The highest dilution that is still positive is reported out as a "1 to dilution rate" ratio (for instance 1:40 or 1:320, etc.). This is still used to report out some antibody levels, especially in the case of autoimmune conditions. "Antibody titer" is a term that is also sometimes used generically to refer to antibody concentrations.)
High levels of individual IgE antibodies may help diagnose an allergy, but they do not necessarily correlate to the severity of the symptoms that a patient may be experiencing. A patient who has been avoiding an offending substance, such as peanuts, may have low to moderate concentrations of IgE peanut antibodies when tested. However, with subsequent exposure, the person’s peanut antibody concentrations would rise again.