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Introduction: Laboratory tests serve a valuable
function in the care of patients with rheumatic disease,
assisting in the diagnosis and monitoring of a number of
illnesses, and also screening for a number of medication side
effects. The following are a list of various laboratory
studies that are useful in treating patients in our office.
This summary is not all-inclusive, but constitutes the most
frequently utilized tests in our office.
Complete
Blood Count (CBC):
This test consists of many components.
Hemoglobin: The main protein
of the red blood cell, carrying oxygen to other parts of the
body.
Red Blood Cells (RBCs): This
and other values are helpful in the assessment of anemia.
Also included are the MCV, MCH, and MCHC, which determine the
size, weight, and hemoglobin content of each red blood cell.
White Blood Cells (WBCs):
These cells are responsible for protecting the body against
infection. Because all of these cells are produced in the
bone marrow, some medications that suppress the bone marrow
may suppress the production of these cells. Conversely, these
cells may be elevated in the presence of infection, certain
inflammatory diseases, or in patients taking corticosteroids.
Hematocrit:
This test combines the number of RBCs and WBCs in the
bloodstream in comparison with the plasma, the amount
of liquid in the bloodstream. This test is helpful in
evaluating anemia and fluid imbalances such as dehydration.
Platelets: These cells are
responsible for clotting the blood. They may be low because
of medication side effects or in certain diseases such as
systemic lupus erythematosus (SLE), or elevated in the
presence of inflammation.
Chemistry Studies:
Comprehensive Metabolic Profile (CMP):
This test includes electrolytes and blood sugar, as well as
liver and kidney function tests. A subset of this test is the
Basic Metabolic Profile, which
does not include liver function studies. Depending on what
medications a patient is taking or what diseases are being
treated, your provider may feel the need to monitor a number
of these blood levels. Specific values listed under these
tests include:
- Electrolytes
Sodium
Potassium
Chloride
- Kidney Function
BUN
Creatinine
- Liver
Function
SGOT (AST)
SGPT (ALT)
Alkaline Phosphatase
- Other
Glucose
Albumin
Muscle Enzymes:
The Creatine Phosphokinase (CPK) and Aldolase
are enzymes found in muscle. These markers are useful both in
the diagnosis of muscular diseases (see Myositis section) and
in monitoring the effectiveness of certain treatments. These
tests can help determine if the cause of a patient�s weakness
is muscular or neurological (due to nerve or brain damage).
Uric Acid: Both
gout (see related section) and certain kidney stones are
caused by crystals formed as a result of increased uric acid
levels. An elevation of the uric acid level is not diagnostic
of gout, as dehydration and certain medications (diuretics,
for example) can contribute to elevated uric acid levels.
Serology
Tests: These
studies are a focus of the rheumatology field and help serve
as markers for many rheumatic diseases. They consist of
either antibodies or other components of the immune system.
Erythrocyte Sedimentation Rate (ESR or
�Sed Rate�) and C-reactive Protein (CRP): These are
non-specific markers for inflammation; i.e. � they can be
elevated in a number of conditions, including not only
inflammatory arthritis but also infections or cancer. We
often use these values, along with physical examination and
the patient�s report, to determine the need for or
effectiveness of certain types of medical treatments.
Complement: A system of
molecules in the immune system, complement proteins are
inactive by themselves, but in certain disease processes this
system can be activated in what is considered a �cascade.� In
the field of rheumatology, the presence of low complement
levels can give us an indication of disease activity and can
gauge the effectiveness of treatment when they improve or
return to normal. On the other hand, the situation becomes
confusing when certain individuals have genetic conditions
where their complement levels are always low regardless
of disease activity.
Serum Protein Electrophoresis (SPEP):
This test involves a process that separates various
proteins fractions into albumin and globulins.
An important set of proteins known as gamma globulins
make up the body�s antibody system which form an important
part of the immune system. This test may demonstrate
abnormal proteins among the gamma globulins that help identify
certain blood diseases such as multiple myeloma and
Waldenstr�m�s disease.
Rheumatoid Factor: This test
is a lab marker for an antibody present in 70-80% of patients
with rheumatoid arthritis and can be a useful piece of
information in making the diagnosis. This marker, however, is
not entirely specific for rheumatoid arthritis; up to 5-10% of
the normal population may also demonstrate this antibody.
Moreover, 20-30% of rheumatoid arthritis patients will not
exhibit this antibody.
Anti-Cyclic Citrullinated Peptide
(Anti-CCP): This test is also a marker for
rheumatoid arthritis. While it is present in a smaller
percentage of patients, when present, it is about 98% specific
for the diagnosis of rheumatoid arthritis and may be helpful
in confirming the diagnosis in borderline cases.
Anti-Nuclear Antibody (ANA):
This antibody is present in about 99% of patients with
systemic lupus erythematosus but is also present in up to
5-10% of the normal population. This antibody can also be
present in patients with drug-induced lupus, Sj�gren�s
syndrome, scleroderma, myositis, juvenile rheumatoid
arthritis, polyarteritis nodosum, and chronic active
hepatitis, among other conditions.
The ANA can be broken down into separate
components, which is called an ANA panel, a series of
more specific antibody tests that may be helpful in narrowing
down the diagnosis. Antibody tests included in the ANA panel
are listed with most common disease associations:
- Anti-Ro/SSA
Sj�gren's
Syndrome
Lupus
- Anti-La/SSB
Sj�gren's
Syndrome
Lupus
- Anti-Smith
Specific for Lupus (+ in 30% of patients)
- Anti-RNP
Lupus
Mixed connective tissue disease
- Anti-Centromere
Limited Scleroderma
Anti-Double Stranded DNA:
Another specific antibody seen in systemic lupus erythematosus
which can function as a marker of disease activity, found in
some ANA panels.
Anticardiolipin and Lupus
Anticoagulant: Subsets of antibodies that interfere
with certain aspects of blood clotting. These tests may be
positive in lupus patients or may be present by themselves and
are associated with blood clots (including strokes) and/or
miscarriages.
Anti-Neutrophil Cytoplasmic Antibody
(ANCA): These are antibodies directed against
proteins in the white blood cells. Depending on the pattern
of the antibody seen, different diseases are suggested by a
positive ANCA. The c-ANCA is associated with Wegener�s
granulomatosis (see Vasculitis section). The presence and
level of this antibody depends on disease severity and
activity. The p-ANCA and �atypical ANCA� may be seen with a
variety of other conditions. These diseases include but are
not limited to Churg-Strauss syndrome, microscopic
polyangiitis, polyarteritis nodosum, systemic lupus
erythematosus, ulcerative colitis, and less commonly Wegener�s.
Some medications, such as hydralazine, propylthiouracil (PTU),
D-penicillamine, and minocycline may produce a positive p-ANCA
or atypical ANCA.
Angiotensin Converting Enzyme
(ACE): The enzyme is found primarily in cells
located in the lungs or the kidneys. It can be elevated in
various pulmonary conditions but for our purposes we look for
it in sarcoidosis. While not entirely reliable, the ACE may
be helpful in diagnosing and gauging the condition of the
lungs in sarcoidosis.
HLA-B27: This is a genetic
marker for ankylosing spondylitis and other diseases known as
spondyloarthropathies (see related sections). It is
present in over 90% of whites and 50-80% of non-whites with
ankylosing spondylitis. This marker can also be present in a
number of people without ankylosing spondylitis, including 8%
of the general population. Most of the diagnosis of this
condition is determined through a history of symptoms,
physical examination, and x-ray findings.
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