While imaging tests can give your doctor a good view of the damage in your knees or hips, laboratory tests are sometimes needed to determine what is causing the damage and how it might be halted.
Arthrocentesis. If you have sudden or unexplained swelling in a knee, hip, or other joint, your doctor may perform an arthrocentesis, removing a little synovial fluid for examination. Excess synovial fluid may indicate infection, crystal deposits, trauma, or inflammation. Before arthrocentesis, the skin is cleaned and an anesthetic spray or injection is used to numb the area. The doctor inserts a needle with a syringe attached into the joint space (you may hear a pop) and withdraws a fluid sample, which is sent to a laboratory for analysis. If you have pain afterward, your doctor may suggest ice and pain-relief medication. The procedure may immediately lessen pain and pressure caused by excess fluid. Knee arthrocentesis can be done in the doctor's office. Hip arthrocentesis is performed by a radiologist, guided by fluoroscopy, a type of x-ray that shows internal structures in motion.
Rheumatoid factor. This blood test detects an antibody present in about 85% of people with rheumatoid arthritis, a systemic autoimmune disease. The same antibody is also present in other medical conditions and in about 3% of healthy people.
Erythrocyte sedimentation and C-reactive protein. These blood tests are general measurements of inflammation of any kind; the higher the result, the more severe the inflammation. Most people with osteoarthritis have normal values, but those who have inflammatory conditions, such as rheumatoid arthritis, usually have elevated levels. High rates may also be an early sign of infection after knee or hip surgery. High levels of C-reactive protein over the long term also indicate an increased risk for heart disease.
Serum uric acid. The serum uric acid test measures uric acid in the blood, which can help diagnose gout, a condition caused by the accumulation of uric acid crystals in a joint.
More Answers from Scott Martin, MD