From WebMD and The Cleveland Clinic:
If your doctor suspects you have arthritis, he or she may have your blood drawn to determine which type of arthritis you have. In people with osteoarthritis, blood tests are not usually abnormal, but with other types of arthritis, including rheumatoid arthritis, certain tests will help your doctor decide which type you have.
This article reviews the different types of blood tests and markers used to diagnose arthritis and other inflammatory-related conditions.