Goals of lab tests

Laboratory tests are often used to evaluate patients with arthritis. These may help determine whether or not a patient has arthritis, and if so, what type; how well he or she is responding to treatment; and to make sure that no serious side effects are occurring.

Even aspirin, which often is the first medication used to treat many types of arthritis, and can be bought at the corner drugstore, can create serious side effects. Some of these side effects are more obvious than others. The more obvious side effects of aspirin and other NSAIDs (nonsteroidal anti-inflammatory drugs) may include ringing in the ears, easy bruising or bleeding, nausea, or vomiting. Other side effects, like kidney or liver damage, may not be as obvious.

Effectiveness of lab tests

It is important to realize that none of the tests are perfect, and none alone will prove that a certain kind of arthritis is or is not present.

About blood tests

A patient with arthritis will have probably more blood tests than any other type of test because blood is the most easily and safely sampled body tissue and contains traces of material from every other part of the body. The most common blood tests include the following.

Hematocrit (HCT) and hemoglobin (Hgb) counts

These measure the number and quality of red blood cells. If you have chronic inflammation, the number of red blood cells usually is low (inflammation is a body process that can result in pain, swelling, warmth, or redness). Low hematocrit and hemoglobin counts may be signs that your medication is causing a loss of blood from your stomach and passing through your bowel. Low counts also may indicate a decrease in red blood cell production.

White blood cell count (WBC)

WBC tests measure the number of white blood cells present. When you have an infection, your body makes more white blood cells. Some medications can decrease the number of white blood cells, which could increase your chances of getting an infection.

Platelet count

This test measures the number of "sticky" cells or platelets that help the blood to clot (aspirin and some other medications make the platelets less "sticky"). If the number of platelets is low you could have too much bleeding. Some powerful arthritis medications in rare instances lower the platelet count.

Erythrocyte sedimentation rate (ESR)

This test, also called "sed rate," determines if you have inflammation. The sed rate can measure the amount of inflammation present. The test measures how fast red blood cells cling together, fall, and settle toward the bottom of a glass tube in an hour's time, like sediment. The higher the sed rate, the greater the amount of inflammation. As inflammation responds to medication, the sed rate usually goes down. This is an example of a test your doctor might order several times. Another test used to measure this is the C-Reactive Protein (CRP) test.

Salicylate level

This measures the amount of salicylate in the blood to find out if enough is being absorbed to reduce inflammation (salicylate is the main ingredient in aspirin and some other NSAIDs). This is a helpful test for people who are taking large doses of these medications for a long time. High salicylate levels can be harmful, with or without such symptoms as ringing in the ears, nausea, vomiting, or even fever.

Muscle enzyme tests (CPK, aldolase)

These tests measure the amount of muscle damage (in some rheumatic diseases, damaged muscles release certain enzymes into the blood). These tests also can show how effective medication has been in reducing inflammation that causes muscle damage.

Liver enzyme tests (SGOT, SGPT, bilirubin, alkalin

These tests measure the amount of liver damage. Certain medications used in the treatment of arthritis can damage the liver. Aspirin, for example, has been known in rare instances to damage the liver, especially in children or in people who have had previous liver damage. Other NSAIDs can cause liver inflammation (hepatitis). Methotrexate can cause liver damage, especially in people who drink alcohol.

Creatinine tests

These measure kidney function. Creatinine is a normal waste product of the muscles. A high level of it means that the kidneys are not working well enough to remove waste products from the body.

What are uric acid tests?

These tests measure the amount of uric acid in the blood. People with gout usually have high levels of uric acid, which forms into crystals that are then deposited in the joints and other tissues. However, not everyone with a high uric acid level has gout.

The immune system

To help you understand why immunological blood tests are performed, following is an explanation of the immune system.

The immune system is the body's natural defense against foreign invaders, such as bacteria or viruses. Occasionally, the immune system breaks down and loses the ability to distinguish between its own body cells and foreign invaders. In autoimmune diseases, the immune system fights its own body cells as if they were invaders. When an invader such as a virus enters the body, the virus creates what is called an antigen. The body's immune system fights the antigen by creating an antibody. When the immune system fights against its own body cells, it creates autoantibodies that attack the body itself. Antinuclear antibodies are autoantibodies that react against the nuclei (cores) of the body's own cells when these cell parts are mistaken for foreign invaders.

Autoimmune diseases

Several types of rheumatic diseases are known as autoimmune diseases. These diseases can be traced to a defect in the body's immune system. Depending on the defect, inflammation may occur in the joints, muscles, internal organs, skin, blood vessels, eyes, or mucous membranes. If your doctor suspects you might have an autoimmune disease, then immunological tests usually are ordered. Such tests include rheumatoid factor, antinuclear antibody, complement, and human leukocyte antigen (HLA) tissue typing.

Rheumatoid factor (RF, Latex)

This measures whether a certain amount of abnormal antibody called rheumatoid factor is in the blood. The majority of people with rheumatoid arthritis (a common disease of inflamed joints that can cause joint alignment problems and loss of function) have a large amount of rheumatoid factor in their blood. However, up to 20 percent of adults with rheumatoid arthritis may never have any rheumatoid factor in their blood. In contrast, about 85 percent of children with juvenile rheumatoid arthritis (ERA) are negative for rheumatoid factor (ERA is a group of diseases, similar to rheumatoid arthritis, that begin in childhood). It is important to note that having a positive rheumatoid factor will assist in the diagnosis, but the test alone is not conclusive.

Methodologies

Latex agglutination testing is still widely used although it is being supplanted by other methods including ELISA and nephelometry that are capable of being done by machine rather than by hand to hopefully improve standardization and reproducibility. Nephelometry uses laser light scatter to measure the formation of immune complexes in this case, rheumatoid factor and human IgG.

Normal range

The latex test is reported in a titer with most labs considering > 1:40 as positive. The nephelometry test is usually reported in international units and the normal range is dependent on the specific laboratory usually < 20 IU.

Utility

Rheumatoid factor is not sensitive nor specific enough to rule in or out rheumatoid arthritis. The rheumatoid factor is present in 70-80% of patients who have RA. This means that 20-30% of patients with RA are seronegative for rheumatoid factor. It is most useful as a prognostic indicator in patients with RA. People with RA who are rheumatoid factor positive typically have a more aggressive disease. It is also useful in confirming one's clinical impression that a polyarthritis that looks like RA is even more likely to be RA. It is also followed in patients with Sjogren's disease to predict the development of lymphoma. Rheumatoid factor production may be a way for the immune system to enlarge immune complexes to make them more easily removed by the spleen and other immune organs.

Antinuclear antibody tests (ANA)

These detect a group of autoantibodies that are found in most people with lupus and scleroderma and in a few people with rheumatoid arthritis. These autoantibodies react with antigens in the nuclei of cells. The antibodies suggest that an autoimmune illness may be present, although many people test positive and have little evidence of serious disease. Specific antinuclear antibody tests are helpful in the diagnosis of certain rheumatic diseases that involve abnormalities in the immune system. The names of the following tests are abbreviations of more complicated-sounding tests. The diseases for which they are used include:

• systemic lupus erythematosus (multiple-system illness, may involve the skin, joints, kidney, etc.); anti-dsDNA, anti-Sm, anti-Ro/SS-A, and antihistone tests help confirm the diagnosis.
• scleroderma (a marked thickening of the skin); the anti-Scl-70 test helps confirm the diagnosis.
• polymyositis (inflammation of muscles, resulting in muscle weakness, sometimes with joint inflammation); anti-Jo-l and anti-PM-l tests may help confirm the diagnosis.
• Sjogren's syndrome (disorder marked by dry eyes and dry mouth); anti-Ro/SS-A and anti-La/SS-B tests may help confirm the diagnosis.
• mixed connective tissue disease (a syndrome with a variety of symptoms, including joint inflammation and swollen fingers); the anti-Ul RNP test helps confirm the diagnosis.

Complement tests

These tests measure the amount of complement proteins circulating in the blood. Complement tests involve the reaction of antibodies with antigens. These tests usually are reserved for diagnosing or monitoring people with active lupus. Those people with lupus frequently have lower-than-normal amounts of complement, especially if the kidneys are affected.

Human leukocyte antigen (HLA) tissue typing tests

These tests detect the presence of certain "genetic markers" or traits in the blood. For example, B-27 is a genetic marker that nearly always is present in people with ankylosing spondylitis (a disease involving inflammation of the spine and sacroiliac joint) and Reiter's syndrome (a disease involving inflammation of the urethra, eyes, and joints). This test also is positive in five to 10 percent of the healthy population.

About urine tests

Several different tests may be done on a urine sample to determine its contents.

The tests show whether the urine contains red blood cells, protein, or a variety of other abnormal substances. The detection of these substances may indicate kidney damage in certain rheumatic diseases, such as lupus. Some medications, such as gold and penicillamine, can cause protein to be lost through the urine.

24-hour urine test

This test evaluates all the urine collected over a 24-hour period. Sometimes the creatinine passed in a 24-hour urine specimen is measured to provide a clearer picture of kidney function than the creatinine blood test. Uric acid, calcium, and protein tests sometimes must also be done on a 24-hour sample (Note: It is very important to collect every drop--a complete collection is needed).

Joint aspiration

Inserting a needle into a joint and aspirating, or removing, synovial fluid from it can provide the doctor with valuable information (synovial fluid is the slippery fluid that fills a joint, providing smoother movement).

Usually this procedure is done in the doctor's office. It generally causes no more pain than drawing blood. An examination of the fluid may reveal what is causing the inflammation, such as uric acid crystals, a sure sign of gout, or bacteria, a sign of infection. If crystals are found, proper medication may be prescribed. If an infection is found, the specific bacteria that are causing it can identified and the most effective antibiotic can be prescribed.

Pain relief

Joint aspiration sometimes can relieve the pain of a badly swollen joint. Usually a corticosteroid is injected through the needle (if an infection is not present) to reduce inflammation for an extended period of time--up to three months in many cases, and for months or years in a few instances. Corticosteroids are a group of drugs related to hydrocortisone, a natural hormone produced in the body. They are NOT the same as the steroids some athletes take. Corticosteroids are very helpful in reducing inflammation.

About biopsies

To help confirm a diagnosis or check on the status of disease activity, your doctor may order a biopsy (or removal of a small piece of tissue) to be examined under a microscope. Three of the most common biopsies include skin, muscle, and kidney biopsies.

Skin biopsies

These are usually done to aid the diagnosis of lupus, vasculitis (inflammation of blood vessels), psoriatic arthritis (inflammation of joints and scaly, inflamed skin), or other forms of arthritis that involve the skin. After using a local anesthetic, a tiny piece of skin is removed.

Muscle biopsies

These are similar to skin biopsies, except the surgeon must go deeper into tissue. Muscle biopsies are used to look for signs of damage to the muscle fibers. This information can help confirm the diagnosis of polymyositis or vasculitis.

Kidney biopsies

These are usually done to check for signs of damage from a disease such as lupus. They are usually done by passing a needle through the back and withdrawing a bit of tissue for examination.

Other biopsies

Other biopsies are done on a less frequent basis. These include synovial, lung, salivary gland, and blood vessel biopsies. Liver biopsies are occasionally done to check for signs of damage in people receiving methotrexate for rheumatoid arthritis.

X-rays for arthritis patients

X-rays enable your doctor to monitor any possible bone damage.

It may take years for bone damage to show up on X-rays, so abnormalities may not always be present on the first set of X-rays. Ankylosing spondylitis is one exception, in which case damage may be apparent early on. Sacroiliac joint X-rays can confirm a diagnosis of ankylosing spondylitis. People with rheumatoid arthritis often have their bones X-rayed because the small bones show the progress of the disease in better detail. Other joints such as the hips, knees, elbows, and feet are X-rayed when the doctor wants to monitor disease activity.

Unnecessary tests

Blood tests, urine tests, joint fluid tests, biopsies, and X-rays are useful tools that your doctor relies on to help diagnose and treat your arthritis. These are by no means all the tests your doctor may order, but they are some of the most common.

Bear in mind that doctors sometimes can order too many tests. Tests should only be done when your doctor needs the results to guide diagnosis or treatment. Unnecessary tests can lead to errors in diagnosis and treatment. Feel free to ask your doctor about tests that might be useful in your case. However, realize that more tests are not always better or even as good as fewer. Your doctor's advice should guide these decisions.

Communicating honestly with your doctor is always important. If you don't understand why a certain test is needed, ask. Your doctor or nurse most likely will gladly explain why it has been ordered.

Credits

Some of this material may also be available in an Arthritis Foundation brochure. Contact the Washington/Alaska Chapter Helpline: (800) 542-0295. If dialing from outside of WA and AK, contact the National Helpline: (800) 283-7800.

This material was originally prepared for the Arthritis Foundation and is protected by copyright.