Blood Tests for Arthritis

Blood tests are used to help diagnose arthritis, monitor treatment effectiveness, and track disease activity. While laboratory blood tests are valuable diagnostic tools, they are not definitive when considered alone. To formulate an accurate diagnosis, the patient's medical history must be evaluated, along with laboratory test results and imaging studies. There are general blood tests and specialized blood tests used to evaluate arthritis.

General Blood Tests

Complete Blood Count (CBC)

The complete blood count is a blood test that counts the number of red blood cells, white blood cells, and platelets. The aforementioned blood components are suspended in plasma (the thick, pale yellow, fluid portion of blood). Automated machines in a laboratory rapidly count the various cell types.

• White Cells: The white cell count is normally between 5,000-10,000 per microliter of blood. Increased values suggest inflammation or infection. Such things as exercise, cold, and stress can temporarily elevate the white cell count.
• Red Cells: Normal values for the red cell count vary with gender. Males normally have values of around 5-6 million red cells per microliter. Females have a lower normal range between 3.6-5.6 million red cells per microliter.
• Hemoglobin and Hematocrit: Hemoglobin, the iron-containing component of red cells which carries oxygen, is also measured in a complete blood count. The normal hemoglobin value for males is 13-18 g/dl. Normal hemoglobin for females is 12-16 g/dl. The hematocrit measures the number of red cells as the percent of total blood volume. Normal hematocrit for males is between 40-55% and the normal hematocrit for females is 36-48%. Generally, the hematocrit is about 3 times the hemoglobin. Decreased values are indicative of anemia. The MCV, MCH, MCHC are red cell indices that indicate the size and hemoglobin content of individual red cells. The indices can provide clues regarding the likely cause of existing anemia.
• Platelets: Platelets are components that are important in clot formation. Many medications used in the treatment of arthritis can decrease the platelet count or affect platelet function. Normal platelet values range from 150,000-400,000 per microliter.
• Differential: The percent and absolute number of each type of white blood cell is called the differential. Neutrophils are increased in bacterial infections and acute inflammation. Lymphocytes are increased in viral infections. Monocytes are increased in chronic infections. Eosinophils are increased in allergies and other conditions. An elevated number of eosinophils is known as eosinophilia. Basophils, which are generally 1 or 2% of the white count differential, rarely are increased.
• Inflammation: The process of inflammation can cause changes in the blood count. The red cell count may go down, the white cell count may go up, and the platelet count may be elevated. While anemia may accompany inflammatory arthritis it may be caused by other things, such as blood loss or iron deficiency. Only when other causes have been ruled out can a doctor interpret blood abnormalities as a sign of inflammation.

Chemistry Panels

The chemistry panel is a series of tests that are used to evaluate key metabolic functions. The group of tests is performed on serum (the portion of blood without cells). Electrolytes, ionized salts in blood or tissue fluids (e.g., sodium, potassium, chloride), are part of a chemistry panel. There are also tests that serve as indicators for heart risk, diabetes, ​kidney function, and liver function.

For example, a patient with a high creatinine level may have a kidney abnormality. Creatinine is a waste product found in the blood. Certain types of inflammatory arthritis can affect kidney function. Certain arthritis drugs can affect kidney function, too. Uric acid is another test that is included in the blood chemistry panel. If elevated, uric acid may be indicative of gout. That is merely a handful of examples. In fact, the chemistry panel provides a lot of information about how the body is functioning.

Specialized Blood Tests

Erythrocyte Sedimentation Rate (ESR)

The erythrocyte sedimentation rate is a test that involves placing a blood sample in a special tube and determining how fast the red blood cells settle to the bottom in one hour. When inflammation is present, the body produces proteins in the blood that make the red cells clump together. Heavier cell aggregates fall faster than normal red cells.

For healthy individuals, the normal rate is up to 20 millimeters in one hour (0-15 mm/hr for men and 0-20 mm/hr for women). Inflammation increases the rate significantly. Since inflammation can be associated with conditions other than arthritis, the sedimentation rate test alone is considered non-specific.

Rheumatoid Factor (RF)

Rheumatoid factor is an antibody found in many patients with rheumatoid arthritis. Rheumatoid factor was discovered in the 1940s and became a significant diagnostic tool in the field of rheumatology. Approximately 80% of rheumatoid arthritis patients have rheumatoid factor in their blood. High concentrations of rheumatoid factor are typically associated with severe disease.

Rheumatoid factor can take many months to show up in the blood. If tested too early in the course of the disease, the result could be negative and re-testing should be considered at a later date. In cases where patients present with signs and symptoms of rheumatoid arthritis but they are seronegative for rheumatoid factor, doctors may suspect that another disease is mimicking rheumatoid arthritis. Rheumatoid factor can also occur in response to other inflammatory conditions or infectious diseases, although usually in such cases, the concentration is lower than with rheumatoid arthritis.

HLA Typing

White blood cells may be typed for the presence of HLA-B27. The test is common in medical centers where transplants are performed. HLA-B27 is also a genetic marker that is associated with certain types of arthritis, mainly ankylosing spondylitis and Reiter's syndrome/Reactive Arthritis.

Antinuclear Antibody (ANA)

The ANA (antinuclear antibody) test is performed to help diagnose certain rheumatic diseases. Patients with certain diseases, especially lupus, form antibodies to the nucleus of the body's cells. The antibodies are called antinuclear antibodies and are detectable by placing a patient's serum on a special microscope slide that contains cells with visible nuclei. A substance containing fluorescent dye is added. The dye binds to the antibodies on the slide, making them visible under a fluorescent microscope.

• Over 95% of patients with lupus have a positive ANA test.
• 50% of rheumatoid arthritis patients are positive for ANA.

Patients with other diseases also can have positive ANA tests. For a definitive diagnosis, other criteria must also be considered.

C-Reactive Protein (CRP)

C-reactive protein measures the concentration of a special type of protein that is produced by the liver. The protein is present in blood serum during episodes of acute inflammation or infection.

As a blood test, CRP is considered non-specific. A high result is indicative of acute inflammation. In cases of inflammatory rheumatic diseases, such as rheumatoid arthritis and lupus, doctors can use the CRP test to monitor treatment effectiveness and disease activity. 

Lupus Erythematosus (LE)

The LE cell test is no longer commonly used. Its initial discovery did open up the whole field of antinuclear antibodies, though. The problem -- only 50% of lupus patients are found to have positive LE tests.

Anti-CCP

Anti-CCP (anti-cyclic citrullinated peptide antibody) is one of the newer blood tests used to confirm the diagnosis of rheumatoid arthritis. If the antibody is present at a high level, it may also suggest that there is a higher risk of severe joint damage.

Anti-DNA and Anti-Sm

Lupus patients form antibodies to DNA (deoxyribonucleic acid). A test is available that checks for the presence of anti-DNA. It is a useful diagnostic tool, especially since anti-DNA is not usually found in people without lupus. The test is also a good monitoring tool because of the levels of anti-DNA rise and fall with disease activity.

Lupus patients also have antibodies to Sm (anti-Smith), another substance in the cell's nucleus. The Sm antibodies also are found only in lupus patients. The test is not particularly useful in monitoring disease activity, though.

Complement

The complement system is a complex set of blood proteins that are part of the body's defense system. The proteins are inactive until an antibody binds to an antigen and activates the complement system. The system produces factors that help destroy bacteria and combat invaders.

These reactions consume complement and leave depressed levels that are indicative of immune complex formation. Lupus patients often show decreased levels of total complement. The complement test may also be helpful in tracking the disease activity of a lupus patient.