Doctors sometimes find it difficult to diagnose reactive arthritis because there is no specific laboratory test to confirm that a person has it.
Blood Tests
A doctor may order a blood test to detect the genetic factor HLA-B27, but even if the result is positive, the presence of HLA-B27 does not always mean that a person has the disorder.
Doctors also may order a blood test to determine the erythrocyte sedimentation rate (sed rate), which is the rate at which red blood cells settle to the bottom of a test tube of blood. A high sed rate often indicates inflammation somewhere in the body. Typically, people with rheumatic diseases, including reactive arthritis, have an elevated sed rate.
Testing for Infection
The doctor also is likely to perform tests for infections that might be associated with reactive arthritis. Patients generally are tested for a Chlamydia infection because recent studies have shown that early treatment of Chlamydia-induced reactive arthritis may reduce the progression of the disease. The doctor may look for bacterial infections by testing cell samples taken from the patient's throat as well as the urethra in men or cervix in women. Urine and stool samples also may be tested. A sample of synovial fluid (the fluid that lubricates the joints) may be removed from the arthritic joint. Studies of synovial fluid can help the doctor rule out infection in the joint.
X-Rays
Doctors sometimes use x rays to help diagnose reactive arthritis and to rule out other causes of arthritis. X rays can detect some of the symptoms of reactive arthritis, including spondylitis, sacroiliitis, swelling of soft tissues, damage to cartilage or bone margins of the joint, and calcium deposits where the tendon attaches to the bone.