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Lupus and Laboratory Tests

 

One of the most typical features of lupus is the presence of particular antibodies in the blood. Antibodies are proteins which recognise and bind to other molecules (usually proteins) in the body. They are usually produced in response to infection. However, instead of getting rid of an unwanted foreign protein (which is what normal antibodies do), the antibodies in lupus recognise components of our own cells (usually DNA or proteins). These antibodies are therefore called auto-antibodies. Why these auto-antibodies are made in lupus is complex and still not fully understood. Binding of the auto-antibody to its target can interfere with the normal function of the target molecules, the cells containing the molecules, or can result in the formation of complexes containing the antibody and its target molecule (called immune complexes) which become trapped in blood vessels and the kidney and cause inflammation and damage. This damage is often due to the activation of a series of proteins called complement (also normally involved in clearing infections from the body). Thus laboratory tests in lupus are performed in order to assess the activity of the disease (for example The type and amount of autoantibodies and complement in the blood), and the effects of the disease and certain drugs used to treat lupus on blood cell counts and blood chemistry.

What is an ANA test?

ANA stands for anti-nuclear antibody. This test detects a group of antibodies directed against components of nucleus of the cell, such as DNA and ribonucleoproteins (RNP). The individual antibodies include anti-DNA antibodies and the various anti-ENA antibodies (see below). The ANA test is used as a screening test for these autoantibodies which may then be identified individually by other tests. The ANA test is positive in 95% of people with lupus but only about 5% of healthy people. It can also be positive in people with related autoimmune conditions (sometimes called connective tissue diseases) such as dermatomyositis, polymyositis, and systemic sclerosis (scleroderma). It is sometimes positive in people with other types of disease such as chronic infection or certain malignancies (cancers). It is therefore not diagnostic of lupus, but it is important supporting evidence when other features (symptoms, signs and other laboratory tests) suggest lupus.

Why is the ANA test done to diagnose lupus?

The ANA test is done as a screening test as it is the commonest laboratory abnormality found in lupus. However, for the diagnosis of lupus to be confirmed it must be associated with several other clinical and/or laboratory features of lupus and no other conditions that can cause a positive ANA. Appropriate clinical features include one of the typical skin rashes, recurrent mouth ulcers, inflammatory arthritis, pleurisy, and epileptic fits, but there are many other possible manifestations. So in anyone suspected of lupus on clinical grounds (symptoms and signs on examination), the ANA test is one of the simplest tests to help establish the diagnosis, as all these clinical problems can result from other conditions. But it must be remembered that certain other conditions can cause a positive ANA test and some healthy people have these antibodies (particularly at low levels in the blood). This explains why it can be hard to diagnose and distinguish lupus from other conditions in the early stages. The diagnosis of lupus becomes more certain as more clinical and laboratory features are found with time.

 
 

 

 
 

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