A diagnosis of reactive arthritis is based on the person's symptoms, medical history, and physical examination. There are a few tests that may be ordered to help make a diagnosis. These include:
- HLA-B27 antigen – to look for a specific protein that is found on cell surfaces; being positive for this protein suggests a greater than average risk for developing certain autoimmune disorders, including reactive arthritis.
- Erythrocyte sedimentation rate (ESR) – to check for inflammation; increased with reactive arthritis as well as other diseases
- C-reactive protein (CRP) – to check for inflammation; increased with reactive arthritis as well as other diseases
Health practitioners may also order tests to identify the infection that triggered the reaction, such as:
- Synovial fluid analysis – to look for an infection in the joint
- Chlamydia test – to look for evidence of infection by the bacterium Chlamydia trachomatis; if this is positive, early treatment can reduce arthritis progression.
- Stool culture – to look for Salmonella, Shigella, Campylobacter, E. coli, Yersinia
- HIV antibody test – to determine if someone is HIV-positive, although it is generally thought that reactive arthritis is related to other infections to which those with HIV have been exposed rather than to HIV infection itself.
A few tests may be ordered to rule out other causes of the symptoms:
- Rheumatoid factor (RF) – to help rule out other diseases, such as rheumatoid arthritis; negative with reactive arthritis
- Antinuclear antibody (ANA) – to help rule out other diseases, such as systemic lupus erythematosus
Non-laboratory tests include X-rays that may be performed to examine the joint for any damage or inflammation or to rule out other types of arthritis.