How useful a serological result is depends on the individual virus.

1. For viruses such as rubella and hepatitis A, the onset of clinical symptoms coincide with the development of antibodies. The detection of IgM or rising titres of IgG in the serum of the patient would indicate active disease.

2. However, many viruses often produce clinical disease before the appearance of antibodies such as respiratory and diarrhoeal viruses. So in this case, any serological diagnosis would be retrospective and therefore will not be that useful.

3. There are also viruses which produce clinical disease months or years after seroconversion e.g. HIV and rabies. In the case of these viruses, the mere presence of antibody is sufficient to make a definitive diagnosis.

There are a number of problems associated with serology:-    

1. long length of time required for diagnosis for paired acute and convalescent sera

2. mild local infections such as HSV genitalis may not produce a detectable humoral immune response

3. Extensive antigenic cross-reactivity between related viruses e.g. HSV and VZV, Japanese B encephalitis and Dengue, may lead to false positive results

4. immunocompromised patients often give a reduced or absent humoral immune response.

5. Patients with infectious mononucleosis and those with connective tissue diseases such as SLE may react non-specifically giving a false positive result

6. Patients given blood or blood products may give a false positive result due to the transfer of antibody.

Complement Fixation Test in Microtiter Plate. Rows 1 and 2 exhibit complement fixation obtained with acute and convalescent phase serum specimens, respectively. (2-fold serum dilutions were used)  The observed 4-fold increase is significant and indicates infection.  

Microplate ELISA: coloured wells indicate reactivity. The darker the colour, the higher the reactivity