What diagnostic test is most suggestive of reactive arthritis?

Reactive arthritis is typically triggered by an infection elsewhere in the body, commonly following a genitourinary or gastrointestinal infection. When considering diagnostic tests for reactive arthritis, a positive urethral swab for Chlamydia trachomatis is highly suggestive of this condition, particularly in young sexually active individuals. The presence of this pathogen may indicate a preceding infection that can lead to inflammatory joint symptoms.

Other tests may be relevant in the diagnosis of various forms of arthritis but do not specifically indicate reactive arthritis. For instance, synovial fluid analysis showing crystals is indicative of gout or pseudogout rather than reactive arthritis. X-ray findings of joint erosion are more associated with conditions like rheumatoid arthritis or osteoarthritis, and lab tests indicating high rheumatoid factor are typically associated with rheumatoid arthritis, not reactive arthritis. Therefore, a urethral swab positive for Chlamydia trachomatis directly connects to the etiology of reactive arthritis and is the most specific diagnostic indicator among the options presented.