More than for most other diseases, the diagnosis and treatment of arthritis require a hands-on approach. By examining your tender joints and muscles and listening to your description of your symptoms and their severity, a doctor can usually get a pretty good idea of what’s going on inside your body.

But there are times when a doctor needs information that only a laboratory examination of bodily fluids and tissues can reveal. When he needs to confirm a diagnosis, monitor disease progress or medication effectiveness, or determine if the drugs you’re taking are causing potentially dangerous – but not evident – side effects, lab tests are in order.

The majority of lab tests are performed on blood because it is easily and safely sampled and it holds many microscopic clues to what’s going on throughout the body. Other tests may require urine, joint fluid or even small pieces of skin or muscle. Whether you’re just beginning the diagnostic process or completing your umpteenth year of treatment, the following information should help you understand some of the most common lab tests you’re likely to encounter.

Making a Diagnosis

While lab tests aren’t needed for every form of arthritis, they are very important to verify and confirm the presence of some diseases, according to Robert Lahita, MD, chief of rheumatology at St. Luke’s/Roosevelt Hospital and associate professor of medicine at Columbia University. If your symptoms and physical examination suggest rheumatoid arthritis, lupus, Sjogren’s syndrome, Lyme disease or one of a few other inflammatory forms of arthritis, the following tests can often confirm your doctor’s suspicions:

Antinuclear antibody (ANA) Commonly found in the blood of people who have lupus, ANAs (abnormal antibodies directed against the cells’ nuclei) can also suggest the presence of polymyositis, scleroderma, Sjogren’s syndrome, mixed connective tissue disease or rheumatoid arthritis. Tests to detect specific subsets of these antibodies can be used to confirm the diagnosis of a particular disease or form of arthritis.

Rheumatoid factor (RF) Designed to detect and measure the level of an antibody that acts against the blood component gamma globulin, this test is often positive in people with rheumatoid arthritis.

Uric acid By measuring the level of uric acid in the blood, this test helps doctors diagnose gout, a condition that occurs when excess uric acid crystallizes and forms deposits in the joints and other tissues, causing inflammation and severe pain.

HLA tissue typing This test, which detects the presence of certain genetic markers in the blood, can often confirm a diagnosis of ankylosing spondylitis (a disease involving inflammation of the spine and sacroiliac joint) or Reiter’s syndrome (a disease involving inflammation of the urethra, eyes and joints). The genetic marker HLA-B27 is almost always present in people with either of these diseases.

Erythrocyte sedimentation rate Also called ESR or “sed rate,” this test measures how fast red blood cells cling together, fall and settle (like sediment) in the bottom of a glass tube over the course of an hour. The higher the rate, the greater the amount of inflammation.

Lyme serology This test detects an immune response to the infectious agent that causes Lyme disease and thus can be used to confirm a diagnosis of the disease.

Skin biopsy Taking small samples of skin and examining them under a microscope can help doctors diagnose forms of arthritis that involve the skin, such as lupus, vasculitis (inflammation of the blood vessels) and psoriatic arthritis.

Muscle biopsy By going a little deeper into the tissue than with the skin biopsy, the surgeon can take a sample of muscle to be examined for signs of damage to the muscle fibers. Findings can confirm a diagnosis of polymyositis or vasculitis.

Joint fluid tests – In this procedure, which is similar to drawing blood, the doctor inserts a needle into a joint space and removes fluid. An examination of the fluid may reveal uric acid crystals, confirming a diagnosis of gout or bacteria, suggesting that the joint inflammation is caused by infection.