19th January 2024

Diagnosis

Investigations

There are no confirmatory tests for reactive arthritis, and diagnosis is based on clinical suspicion in a patient with inflammatory arthritis and a history of a preceding genitourinary or gastrointestinal infection. Investigations are often performed to rule out other diagnoses.

Blood tests:

• FBC: may reveal normocytic normochromic anaemia, mild leucocytosis, and thrombocytosis in the acute stage
• ESR and CRP are usually very high in the acute stage
• HLA B27 is positive in most affected patients; however, is not specific for reactive arthritis
• Rheumatoid factors and antinuclear antibodies are negative

Identifying causative organism:

• Urogenital and stool cultures: positive at the time of infection, but are usually negative post onset of arthritis
• Men: Urine nucleic acid amplification test (NAAT) for C. trachomatis and N. gonorrhoeae and urethral culture for N. gonorrhoeae
• Women: Vulvovaginal NAAT for C. trachomatis and N. gonorrhoeae and endocervical culture for N. gonorrhoeae 
• Both sexes: HIV, syphilis, hepatitis B and hepatitis C screening depending on clinical history
• Serology for Yersinia, Campylobacter, Salmonella and Shigella if indicated

Joint aspiration

• Joint aspiration may be needed to rule out septic arthritis or crystal arthropathy.
• Synovial fluid in patients with reactive arthritis would demonstrate a high white cell count (2000-50,000 cells/mm3. ) with a neutrophil count above 50% and may reveal triggering organisms, e.g. gonorrhoea.

Imaging

• X-rays: may be normal in early disease or show non-specific changes such as soft tissue swelling. In advanced disease, joint space narrowing and erosions in the joints of the hands and feet may be seen. X-rays may also reveal bilateral but asymmetrical sacroiliitis or enthesitis as fluffy periosteal new bone formation. Non-marginal syndesmophytes are typically seen.
• MRI: may be more sensitive in detecting sacroiliitis than x-rays in patients with lower back pain
• Ultrasound: may detect synovial hypertrophy and increased vascularity using doppler imaging