Doctors sometimes find it difficult to diagnose reactive arthritis because there is no specific laboratory test to confirm that a person has it. A doctor may order a blood test to detect the genetic factor HLA-B27, but even if the result is positive, the presence of this gene does not always mean that a person has the disorder.

 

The doctor may use various blood tests besides the HLA-B27 test to help rule out other conditions and confirm a suspected reactive arthritis diagnosis. For example, the doctor may order rheumatoid factor or antinuclear antibody tests to rule out reactive arthritis. Most people who have reactive arthritis will have negative results on these tests. If a person's test results are positive, he or she may have some other form of arthritis, such as rheumatoid arthritis or lupus.

 

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.

 

The doctor also is likely to perform tests for infections that might be associated with reactive arthritis. People 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 person's throat, as well as from the urethra in men or the 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.