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.