Patients with arthritis will probably have blood tests when they first see the doctor, and as part of their 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 help diagnose and manage arthritis include the following:

Complete blood count — The complete blood count (CBC) is a series of blood tests that provides information about the different parts 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 also tell how serious the disease is.

Under normal conditions, the white blood cell count is between 4,000 and 11,000. A high white blood cell count could mean there is inflammation (swelling), which can be caused by rheumatoid arthritis (RA). However, infections, stress, and exercise will temporarily raise the white blood cell count, too.

A CBC also measures hemoglobin, a component of red cells that contains iron and carries oxygen. Thehematocrit is the percent of total blood volume that is made up of red cells. Normal hematocrit values are 39 to 51% for males, and 36 to 46% for females. A lower hematocrit can be caused by a number of factors or conditions, including RA.

Erythrocyte sedimentation rate — The erythrocyte sedimentation rate (ESR) is a test in which a blood sample is placed in a tube to see how far the red blood cells settle in one hour. Inflammation in the body produces proteins in the blood, which make the red cells clump together and causes them to fall faster than the healthy blood cells. Since inflammation can be caused by conditions other than arthritis, the ESR test alone does not diagnose arthritis.

Rheumatoid factor — Rheumatoid factor (RF) is an antibody found in many patients with RA. It is one of several methods used to diagnose RA (80 percent of RA patients have RF in their blood, though other inflammatory or infectious diseases may also be the cause).

Antinuclear antibody — 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 (ANAs), are discovered by viewing the patient's blood serum (clear liquid separated from the blood) under a microscope. 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 diagnosis can be made.