Lab Test Guide
Learn what your doctor is looking for when he orders a vial of blood or draws fluid from an inflamed knee.
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.
Anti-cyclic citrullinated peptide (anti-CCP) -- Also called anti-citrullinated protein antibodies (ACPA), this test (like the test for rheumatoid factor) looks for the presence of a particular autoantibody that is present in approximately 60-80 percent of people with RA. While most patients with anti-CCP antibodies are also positive for rheumatoid factor, the RF antibody can occur in patients with many other conditions, including an infection. Anti-CCP is more specific for RA and is becoming the preferred test.
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 reactive arthritis (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 sed rate, the greater the amount of inflammation. There are many conditions that can cause an elevated ESR, including an infection or anemia.
C-reactive protein – This test, also called CRP, is another blood test that measures body-wide inflammation. It measures a substance produced by the liver that increases in the presence 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; it can also reveal the presence of other types of crystals. Bacteria cultured from joint fluid can demonstrate that the joint inflammation is caused by an infection.
Monitoring Disease Severity and Medication Response
To determine the progression of the disease or how it is responding to treatment, doctors sometimes use some of the same tests they use to diagnose arthritis. For example, a joint fluid test may show that an infectious agent has been eradicated by antibiotics. Or a “sed rate” test may be conducted a number of times to determine if inflammation is subsiding.
Salicylate level – This test measures the amount of salicylate (the main ingredient in aspirin and in some other NSAIDs) in the blood to determine if enough is being absorbed to effectively reduce inflammation. The test can also determine if the level of salicylate is high enough to create dangerous side effects. This test is not typically used today unless a patient is on high doses of aspirin.
Muscle Enzyme tests (CPK, aldolase) – Muscles that have been damaged by some rheumatic diseases release certain enzymes into the blood, and these enzymes can be detected through blood tests. Such tests can measure the amount of muscle damaged as well as how effective medication has been in reducing the inflammation that caused the muscle damage.
Creatinine test – This test is used to determine the extent of kidney function by measuring the level of creatinine, a normal waste product of the muscles, in the blood. A test showing a high level of creatinine means that the kidneys are not working well enough to remove waste products from the body. Doctors may use this test to monitor kidney function in people with lupus or in those taking medications that could affect the kidneys.
Checking for Drug Side Effects
Often a drug side effect is obvious – you become nauseated, develop a rash or experience blurred vision or ringing in your ears. In other cases, the effects progress unnoticed – until a liver is damaged or a silent ulcer begins to bleed dangerously. For that reason, doctors often use lab tests to check for side effects – before they become major problems. The following tests are the most common:
Liver enzyme tests (SGOT, SGPT, bilirubin, alkaline phosphatase) – These tests, which measure levels of liver enzymes in the blood, can help doctors determine if certain medications have caused damage to the liver.
Hematocrit (HCT) and hemoglobin (Hgb) – These tests measure the number and quality of your red blood cells. Low counts may suggest that your medications are causing gastrointestinal bleeding.
White blood cell count – A blood test showing a low number of infection-fighting white blood cells may suggest that your medication is decreasing your supply of white blood cells and, thus, your body’s chances of fighting infection.
Platelet count – This test measures the number of platelets, or “sticky” cells, that help the blood to clot. A low number of platelets could suggest that your medication has put you at risk of bleeding heavily.
Lab Tests’ Limitations
Despite their many benefits, lab tests have their limitations. Some may show negative results even when a person has the disease being tested for; others may be positive in people who don’t have – or may never develop – a particular disease. In the early stages of rheumatoid arthritis, for instance, only one in five people tests positive for rheumatoid factor; about 8 percent of people without ankylosing spondylitis have the HLA-B27 genetic marker.
Not all forms of arthritis can be confirmed by lab tests. Other tests, including X-rays, various types of angiograms (studies of blood vessels) and magnetic resonance imaging (MRI), may be required to diagnose osteoarthritis, determine the cause of chronic back pain, or examine internal organs affected by some forms of arthritis.
Even so, lab tests are essential to the diagnostic and treatment process. The right tests, along with your doctor’s own observations and your participation in the process, can help you get the safest and most effective treatment for your disease.