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Learning topic 7: Serological tests

Serological tests used to identify viral infections

Many of these tests can be manipulated so they enable the identification of either:

• the virus itself — isolated from appropriate clinical specimens such as blood, CSF or faeces — by the use of antisera of known specificity, such anti-polio type1 sera; or
• a rise in titre (at least four-fold) of specific antiviral antibodies produced by the patient in the course of of a specific viral infection — by the use of known viral cultures or viral antigens. IgM antibodies are the first to appear (10 days) but usually drop below detectable levels within 3 to 4 months; IgG antibodies rise more slowly but remain elevated for many years.

Obviously, if rapid diagnosis is required, it is preferable to try to isolate and identify the virus itself, since it will take at least 10 days post-infection for the patient to produce a specific humoral immune response which can be identified in the laboratory. For this reason diagnosis of viral infections by identification of specific antibody responses is often termed "retrospective" since, with the exception of some chronic diseases caused by viruses such as HIV and Hepatitis B, the patient has usually recovered from the illness by the time diagnosis is confirmed.

The table below shows a summary of common serological tests used to identify viral infections:

Procedure	Principle	Sensitivity	Specificity
Virus neutralisation	Specific antibody neutralises viral infectivity in cell cultures or animal inoculations	High	Very high
Haemagglutination Inhibition*	Specific antibody inhibits the agglutination of RBCs by a virus by binding to viral haemagglutinin	High	High
Complement fixation	A two-stage test: (1) antigen-antibody complexes bind complement, which is then unavailable for (2) the lysis of sheep RBCs by 'haemolysin' (anti- sheep RBC antibodies)	Moderate	Moderate
Immunofluorescence	Tissue sections, smears or cell cultures are tested wit with a specific anti-viral antibody which has been labelled with a dye (fluorescein) which fluoresces under UV light	Low to moderate	Moderate to high
Radioimmunoassay	Specific antibody (or antigen) can be labelled with a radioactive isotope: formation of complexes with antigen (or antibody) in specimens is measured by scintillation counting	Very high	High
ELISA (enzyme-linked immunosorbent assay)	Specific antibody (or antigen) can be labelled with an enzyme: formation of complexes with antigen or antibody in specimens is measured by the use of substrates which form coloured compounds in the presence of the enzyme	High	High
Immunoelectron- microscopy	Specific antibody aggregates the virions which can be visualised under very high magnifications (e.g.  x 20,000)	Very low	High

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