Diagnostic Studies - Diagnosing Arthritis: Arthritis

Content provided by the Faculty of the Harvard Medical School

Diagnostic studies

In most types of arthritis, laboratory tests and x-rays or other imaging techniques may be helpful, but by themselves rarely provide enough information for doctors to establish a specific diagnosis. However, there are exceptions. A bacterial infection of the joint, gout, and pseudogout can be diagnosed by removing and testing a sample of joint fluid (see "Arthrocentesis"). X-rays are occasionally diagnostic as well. For example, x-ray abnormalities in the pelvis and spine may reveal ankylosing spondylitis (see "Diagnosing ankylosing spondylitis").

Blood tests

Doctors often order a complete blood cell count and blood chemistry tests to look for evidence of systemic diseases, including anemia and infection.

Antibody tests. These tests detect various antibodies whose presence may suggest particular types of arthritis. When rheumatoid arthritis is a possibility, many doctors order a test for rheumatoid factor, an antibody that's present in 70%–80% of people with rheumatoid arthritis. But the test is not definitive, so often a second, newer blood test, the anticitrullinated cyclic protein (anti-CCP) test, will usually be ordered as well (see "Blood tests for rheumatoid arthritis"). Antibody tests also exist for other types of arthritis. When lupus is a consideration, for example, doctors will often order a blood test to detect antinuclear antibodies (ANAs).

Erythrocyte sedimentation rate and blood level of C-reactive protein. These blood tests are general measurements of inflammation of any kind: The higher the result, the more severe the inflammation. Most people with osteoarthritis have normal values, but those who have inflammatory conditions, such as rheumatoid arthritis, usually have elevated levels (see "Blood tests for rheumatoid arthritis").

Serum uric acid test. This test measures the level of uric acid in the blood, which is usually elevated in people with gout (see "Diagnosing gout").

Other blood tests. A person's history may indicate the need to test for Lyme disease or other infections, which can cause reactive arthritis and other types of infectious arthritis.

Imaging tests

Doctors may order one or more imaging tests to better evaluate your joints. The type of test ordered depends on the suspected diagnosis.

X-rays. Most forms of arthritis can cause joint abnormalities that are detectable on x-ray examination (see Figure 4). But in most cases, such changes can't be detected until months after the onset of the disease. Sometimes the changes are reasonably specific and suggest a particular kind of arthritis. In other cases, they are more general. For example, bone damage (called erosion) is often found in rheumatoid arthritis and may occur in gout, but the damage from each cause differs enough in appearance that a radiologist can often tell them apart.

Figure 4: Osteoarthritis of the hip This x-ray, or radiograph, shows osteoarthritic changes of the left hip. The normal "ball-in-socket" shape has noticeably deteriorated.

Often, the changes revealed in x-rays bear little relationship to the actual symptoms, especially in osteoarthritis. An x-ray showing large bone spurs on the finger joints may belong to a woman with occasional mild aching in her hands, while an x-ray revealing much less dramatic abnormalities may be that of a woman who can no longer garden because of hand pain.

In their early stages, osteoarthritis and rheumatoid arthritis may appear quite different on x-ray examination, but later they may look similar. In rheumatoid arthritis, the pannus (inflamed tissue) erodes cartilage, and in many cases, the joint damage eventually leads to secondary osteoarthritis, even after the inflammation subsides.

Magnetic resonance imaging (MRI). In evaluating patients with joint problems, this test is helpful to assess soft tissues, cartilage, tendons, and joint inflammation. It's also quite good for spinal cord and nerve root compression that can be caused by spondylitis or degenerative disk disease. MRI has been used to help diagnose rheumatoid arthritis, although some fear it is being overused in this regard (see "Diagnosing rheumatoid arthritis").

Scintigraphy. Another technique for detecting bone abnormalities is scintigraphy, usually done as a whole-body scan several hours after a special radioisotope (a radioactive dye) is injected into a vein. Scintigraphy does not provide as much specific information as MRI, but it can be useful for detecting bone disease.

Other tests. Researchers are studying the ability of ultrasound to detect erosions in rheumatoid arthritis and other types of arthritis. This type of test uses sound waves to assess fluid in soft tissues and abnormalities in muscles or tendons.

Doctors occasionally order computed tomography (CT) scans to evaluate joints for hidden fractures, torn cartilage, and other structural abnormalities. CT imaging uses a rotating x-ray tube housed in a doughnut-shaped machine to take thin-slice x-rays of your anatomy. A computer then assembles these slices into a three-dimensional picture.

Arthrocentesis

People with sudden or unexplained joint swelling may undergo arthrocentesis, in which a physician removes some of the synovial fluid for examination. Excess synovial fluid may indicate a bacterial infection in the joint, crystal deposits, injury, bleeding into the joint, or synovial inflammation. In cases of relatively mild chronic arthritis, arthrocentesis may help distinguish between osteoarthritis and inflammatory joint disease; this can help to narrow down the diagnostic possibilities and guide treatment.

This procedure can be done in the doctor's office and only takes a few minutes. First the skin over the joint is cleaned and an anesthetic agent (typically given by injection under the skin or as a spray) is used to numb the area. The doctor then inserts a needle through the numbed area into the joint space and withdraws some synovial fluid.

Physicians can often get a good idea of whether the problem is inflammatory by the appearance of the fluid. Normally it's translucent and pale-to-medium yellow. Significant inflammation may produce a deep yellow or greenish-yellow opaque fluid. Cloudy fluid may be a sign of crystals or infection.

Laboratory technicians examine the fluid under a microscope for crystals that indicate gout or similar disorders. Your doctor often requests other laboratory tests on the fluid, such as a white blood cell count; a large number of white blood cells could indicate either infection or severe inflammation. Arthrocentesis itself is often beneficial because removing some of the excess synovial fluid can relieve pain and pressure.

Last updated:	September 05, 2008