Arthritis Treatment: What Are the Common Tests Ordered on Arthritis Patients?

Laboratory tests are an integral part of "working up" a patient with arthritis. While there is no substitute for a careful and thoughtful history and physical examination, laboratory testing has a number of important functions including screening in and out different conditions, confirming diagnostic suspicions, staging disease, monitoring progress of disease and checking on effects of therapy.

While routine testing including complete blood count (CBC), chemistries, urinalysis, and such are part and parcel of "standard" testing, this article will focus on the "arthritis tests' most often ordered.

The erythrocyte sedimentation rate (sed rate) is a time honored blood test for measuring inflammation. As a response to inflammation, the liver produces proteins that coat red blood cells. This causes the blood cells to clump and sediment faster than individual red blood cells. The sed rate is sensitive but not very specific, so that it can be increased as a result of inflammation due to arthritis, malignancy, or infection. Other conditions where the sed rate can be elevated are pregnancy and diabetes.

The C-reactive protein (CRP) is another blood test that measures inflammation. It rises more quickly than the sed rate and resolves more quickly as well. In that respect it probably is a better test for monitoring the inflammation of arthritis. Unfortunately it can also be elevated in patients with other conditions such as heart disease, diabetes, cigarette smoking, and obesity.

Rheumatoid factor is an antibody (IgM) that binds to another antibody (IgG). It is positive in about 70-75 per cent of patients with rheumatoid arthritis. Early in the disease it can be negative and it remains negative in about 20 per cent of patients with rheumatoid arthritis. It can also be positive in other diseases such as syphilis, sarcoidosis, malignancies, and infections. In recent years, other forms of rheumatoid factor have been identified which can be useful in diagnosing patients who have rheumatoid arthritis but don't fit the "usual mold."

Anti-CCP is another blood test that is based on an autoantibody directed against the amino acid, arginine. It is much more specific than rheumatoid factor for rheumatoid arthritis although in our lab it is a bit less sensitive. Anti- CCP can occur early in the disease and predicts patients who will have more aggressive disease. We often will use the combination of rheumatoid factor and anti-CCP to achieve both increased sensitivity and specificity for diagnosis.

Antinuclear antibodies are just that... antibodies directed against the nucleus of cells. This is a test that is useful for screening for systemic lupus erythematosus. A patient who is negative for ANA does not have lupus with about 99 per cent certainty. Unfortunately, ANA can be positive in many other conditions including other autoimmune diseases such as Sjogren's disease, scleroderma, inflammatory muscle disease, malignancy, and infection. It can also be positive in up to 30 per cent of healthy people.

For patients where a high level of ANA is noted, there are more specific tests that can be ordered in order to sort out the diagnosis.