Diagnosing a Viral Infection

Laboratory investigations

Clinically it is very difficult to differentiate between a bacterial and viral infection.  Hence laboratory tests help in diagnosing viral infection.  By having some knowledge about what these tests are and how they are done, you will understand why Doctors request these tests and what exactly needs to be seen in them.

Methods of testing include-

         Haematology

         Serology

         Isolation of the virus

         Direct demonstration

Haematalogy

A blood sample is taken to see the full blood count.  Here the total and  the breakdown of the different groups of cells that make up the white blood cell family in seen.  White blood cells are made up of Neutrophils, Lymphocytes, Monocytes and Eosinophils.  An increase or decrease in the total count with and increase in the Lymphocyte is count indicates a viral infection.

Serology

Demonstrating antibodies against a virus is serology.  It’s the commonest and widely used method in practice today.

There are many antibodies which are formed against the invading virus.  They are known as IgM and IgG etc.  IgM is the first to appear in response to the infection. Demonstrating a high level of antibody, once only does not indicate a viral infection. Antibodies may be present in blood even years after an infection. Identifying IgM antibody as well as showing that the levels keep on increasing through out an infection is the way to diagnose  a viral infection.

Different tests used in serology

Many tests are done to see the antibodies.  A few of them are as follows -

             The ELIZA test

             The RIA test

             Complement fixation test

             Immunoflurosence

             Haemagglutination inhibition test,

ELIZA test

ELIZA stands for enzyme linked immuno absorbent assay. Now it is even available as a commercial kit.  The principle here is as follows. A sample of infected blood contains antibodies against the virus. Into this sample an enzyme labeled antihuman IgM antibody is added.  This will react with the previous antibody.  There is also another substrate added which will indicate this reaction by a colour change.

RIA

RIA stands for radio immuno assay.  The same principle as ELIZA is used but the detecting antihuman antibody  IgM is tagged with a radioactive isotope.

Complement fixation test

An antibody  is formed against a virus which in turn captures the virus.  This process is helped by factors called complement.  These are part of the normal immune mechanism against an invading foreign particle.  This process is called fixation.  Fixation is rendered visible by addition of sheep red blood cells  which are sensitized by addition of anti red blood cell anti body.  If fixation has taken place sheep red blood cells donot disintegrate.

Immunofuroscence

Virus antibody is detected on the cells by application of fluorescent labeled anti human antibody.  Fluorescence is detected by microscope under ultra violet light.

Virus isolation

Isolation and growth of microorganisms are usually done in culture mediums under laboratory conditions, in test tubes and Petri dishes.  A virus cannot be grown in normal culture mediums.  It needs special living cell culture mediums as it is an inactive organism which becomes active only inside a living cell.  Several types of cell cultures are used for this purpose.

Primary culture media- cells forming this culture are taken from the monkey kidney.  This is not a very useful medium as the cells die in two to three weeks and also     have very little cell division taking place.

Semicontinous culture- Here the cells come from the human embryo lung.  These cells are susceptible to a wide range of diseases.  They divide as much as thirty to forty times before dying.

Continuous cell line culture-

These cells taken form human cervical cancers divide indefinitely, but the disadvantage is they re susceptible to only a few diseases.

When the virus has replicated and grown in the culture media it is demonstrated by immunoflurosecnce and methods to identify cells that have disintegrated.

Direct demonstration

The fastest method of diagnosing is to see it under electron microscopy.

Another method known as antibody capture test uses monochlonal radioactive labeled antibodies.  This is a superior version of both ELIZA and RIA.  Here the virus reacting with  the antibody is captured and by adding radioactive antihuman antibody the radioactivity in the counter is calculated.

All these tests are done with the primary objective of early diagnosis, so that treatment is initiated early to prevent mortality and morbidity