What determines if serological test results are diagnostic when drawn from a patient for a viral disease?

For serological tests used to diagnose viral diseases, determining diagnostic results often relies on measuring changes in antibody titers over time. The second antibody titer being at least four times greater than the first indicates an increase in antibody levels, which is typically suggestive of a recent or active infection. This fourfold rise strongly supports the diagnosis because it reflects the patient's immune response to the viral antigen.

In contrast, the first antibody titer alone may not provide sufficient information about the timing of infection or the immune response, as it can indicate past exposure but not necessarily current infection. Presence of a visible reaction can be helpful, but it does not quantify the immune response or confirm active disease. The presence of IgM antibodies is also an important marker for recent infections, but it is not the sole determinant; therefore, the fourfold increase in antibody titer is a more definitive hallmark of an ongoing immune response indicative of a current infection.