Patients with arthritis will probably have blood tests as part of their
initial evaluation and follow-up care. This is because blood is the most easily
and safely sampled body tissue, and it contains traces of material from every other
part of the body. The most common blood tests used to assist in the diagnosis
and management of arthritis include:
Complete blood count (CBC) — The complete blood count is a series of
blood tests that provides information about the components of blood including
red blood cells, white blood cells, and platelets. Automated machines rapidly
count the cell types. The CBC test results can help diagnose diseases and
determine their severity.
Under normal conditions, the white blood cell count is between 5,000 to10,000.
A high white blood cell count could suggest inflammation, which can be due to
rheumatoid arthritis (RA). However, infections, stress, and exercise will
temporarily elevate the white blood cell count, too.
A CBC also measures hemoglobin, the iron-containing component of red cells
that carries oxygen. The hematocrit is the percent of total blood volume that is
made up of red cells. Normal value for males is 40 percent to 55 percent, and 36
percent to 48 percent for females. A lower hematocrit can be caused by a number of factors or
conditions including rheumatoid arthritis.
Erythrocyte sedimentation rate (ESR) — The erythrocyte sedimentation
rate is a test that involves placing a blood sample in a tube and determining
how far the red blood cells settle in one hour. When there is inflammation in
the body, it produces proteins in the blood, which make the red cells clump
together, causing them to fall faster than the healthy blood cells. Since
inflammation can be caused by conditions other than arthritis, the ESR test
alone is not diagnostic of arthritis.
Rheumatoid factor (RF) — Rheumatoid factor is an antibody
found in many patients with rheumatoid arthritis. It is one of several criteria
used in diagnosing RA, as 80 percent of RA patients have RF in their blood. A RF test can be positive in
response to other inflammatory or infectious diseases other than RA.
Antinuclear antibody (ANA) — Patients with certain rheumatic
diseases, such as lupus, make antibodies that are directed at the nucleus
of the body's cells. These antibodies, known as antinuclear antibodies, are
detected by viewing the patient's blood serum (clear liquid separated from the
blood) under a microscope. A substance containing fluorescent dye that causes
the antibodies to bind is then added to the serum. This allows the abnormal
antibodies to be seen binding to the nuclei. More than 95 percent of patients with
lupus have a positive ANA test. However, patients with other diseases also can have positive ANA test results, and even perfectly healthy people can have positive ANA test results, so other
tests must be completed before a definitive diagnosis can be given.