Laboratory diagnosis of viral infections

Overview
In the diagnostic laboratory virus infections are confirmed by several methods that include:
 * Growth of the virus in a cell culture from a specimen taken from the patient.
 * Detection of virus-specific antibodies in the blood.
 * Detection of virus antigens
 * Detection of virus nucleic acids
 * Observation of virus particles by electron microscopy.
 * Hemagglutination assay

Cell culture
When growing virus in a cell culture, the cells affected with virus will evolve morphologic changes, often specific for the type of virus involved.

Antibody detection
When the adaptive immune system of a vertebrate encounters a virus, it produces specific antibodies which bind to the virus and render it non-infectious. This is called humoral immunity. Two types of antibodies are important. The first called IgM is highly effective at neutralizing viruses but is only produced by the cells of the immune system for a few weeks. The second, called, IgG is produced indefinitely. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past. Both types of antibodies are measured when tests for immunity are carried out.

Antigen detection
Detection of virus antigens can be done by ELISA in tissues and fluids.

Other techniques are:
 * immunofluorescence
 * immunoperoxidase

Hemagglutination assay
Many viruses attach to molecules present on the surface of red blood cells. A consequence of this is that - at certain concentrations - a viral suspension may bind together (agglutinate) the red blood cells thus preventing them from settling out of suspension. Usefully, agglutination is rarely linked to infectivity, attenuated viruses can therefore be used in assays.

Nucleic acid detection
Detection of virus encoded DNA and RNA is done with polymerase chain reaction. Nucleic acid hybridization with virus-specific probes detects specific viruses.