Lab Tests.
Last updated Tuesday, January 04, 2005
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Immunological blood tests
What is the immune system?
To help you understand why immunological blood tests are performed, following is an explanation of the immune system.
The immune system is the body's natural defense against foreign
invaders, such as bacteria or viruses. Occasionally, the immune system
breaks down and loses the ability to distinguish between its own body
cells and foreign invaders. In autoimmune diseases, the immune system
fights its own body cells as if they were invaders. When an invader
such as a virus enters the body, the virus creates what is called an
antigen. The body's immune system fights the antigen by creating an
antibody. When the immune system fights against its own body cells, it
creates autoantibodies that attack the body itself. Antinuclear
antibodies are autoantibodies that react against the nuclei (cores) of
the body's own cells when these cell parts are mistaken for foreign
invaders.
What are autoimmune diseases?
Several types of rheumatic diseases are known as autoimmune diseases.
These diseases can be traced to a defect in the body's immune system.
Depending on the defect, inflammation may occur in the joints, muscles,
internal organs, skin, blood vessels, eyes, or mucous membranes. If
your doctor suspects you might have an autoimmune disease, then
immunological tests usually are ordered. Such tests include rheumatoid
factor, antinuclear antibody, complement, and human leukocyte antigen
(HLA) tissue typing.What is a rheumatoid factor (RF, Latex) blood test? What is a normal range for rheumatoid factor test results?
This measures whether a certain amount of abnormal antibody called
rheumatoid factor is in the blood. The majority of people with rheumatoid arthritis
(a common disease of inflamed joints that can cause joint alignment
problems and loss of function) have a large amount of rheumatoid factor
in their blood. However, up to 20 percent of adults with rheumatoid
arthritis may never have any rheumatoid factor in their blood. In
contrast, about 85 percent of children with juvenile rheumatoid arthritis (ERA)
are negative for rheumatoid factor (ERA is a group of diseases, similar
to rheumatoid arthritis, that begin in childhood). It is important to
note that having a positive rheumatoid factor will assist in the
diagnosis, but the test alone is not conclusive.
Methodologies
Latex agglutination testing is still widely used although it is
being supplanted by other methods including ELISA and nephelometry that
are capable of being done by machine rather than by hand to hopefully
improve standardization and reproducibility. Nephelometry uses laser
light scatter to measure the formation of immune complexes in this
case, rheumatoid factor and human IgG.
Normal range
The latex test is reported in a titer with most labs considering
> 1:40 as positive. The nephelometry test is usually reported in
international units and the normal range is dependent on the specific
laboratory usually < 20 IU.
Utility
Rheumatoid factor is not sensitive nor specific enough to rule in or
out rheumatoid arthritis. The rheumatoid factor is present in 70-80% of
patients who have RA. This means that 20-30% of patients with RA are
seronegative for rheumatoid factor. It is most useful as a prognostic
indicator in patients with RA. People with RA who are rheumatoid factor
positive typically have a more aggressive disease. It is also useful in
confirming one's clinical impression that a polyarthritis that looks
like RA is even more likely to be RA. It is also followed in patients
with Sjogren's disease to predict the development of lymphoma.
Rheumatoid factor production may be a way for the immune system to
enlarge immune complexes to make them more easily removed by the spleen
and other immune organs.
What are antinuclear antibody tests (ANA)?
These detect a group of autoantibodies that are found in most people with lupus and scleroderma and in a few people with rheumatoid arthritis.
These autoantibodies react with antigens in the nuclei of cells. The
antibodies suggest that an autoimmune illness may be present, although
many people test positive and have little evidence of serious disease.
Specific antinuclear antibody tests are helpful in the diagnosis of
certain rheumatic diseases that involve abnormalities in the immune
system. The names of the following tests are abbreviations of more
complicated-sounding tests. The diseases for which they are used
include:
- systemic lupus erythematosus
(multiple-system illness, may involve the skin, joints, kidney, etc.);
anti-dsDNA, anti-Sm, anti-Ro/SS-A, and antihistone tests help confirm
the diagnosis.
- scleroderma (a marked thickening of the skin); the anti-Scl-70 test helps confirm the diagnosis.
- polymyositis
(inflammation of muscles, resulting in muscle weakness, sometimes with
joint inflammation); anti-Jo-l and anti-PM-l tests may help confirm the
diagnosis.
- Sjogren's syndrome (disorder marked by dry eyes and dry mouth); anti-Ro/SS-A and anti-La/SS-B tests may help confirm the diagnosis.
- mixed
connective tissue disease (a syndrome with a variety of symptoms,
including joint inflammation and swollen fingers); the anti-Ul RNP test
helps confirm the diagnosis.
What are complement tests?
These tests measure the amount of complement proteins circulating in
the blood. Complement tests involve the reaction of antibodies with
antigens. These tests usually are reserved for diagnosing or monitoring
people with active lupus. Those people with lupus frequently have lower-than-normal amounts of complement, especially if the kidneys are affected.What are human leukocyte antigen (HLA) tissue typing tests?
These tests detect the presence of certain "genetic markers" or traits
in the blood. For example, B-27 is a genetic marker that nearly always
is present in people with ankylosing spondylitis (a disease involving inflammation of the spine and sacroiliac joint) and Reiter's syndrome
(a disease involving inflammation of the urethra, eyes, and joints).
This test also is positive in five to 10 percent of the healthy
population.
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