Reactive arthritis

Reactive arthritis is a form of inflammatory arthritis triggered by an infection in another part of the body, usually a bowel infection (from certain food-poisoning bacteria) or a sexually transmitted infection, that occurred one to four weeks earlier. The joints themselves are not infected — instead the immune system, having reacted to the infection, mistakenly inflames the joints. It belongs to the spondyloarthritis family and is more common in people who carry HLA-B27. It classically causes an asymmetric arthritis of a few large joints in the lower limbs — knees, ankles and the small joints of the feet — often with swelling of a whole finger or toe (dactylitis) and pain where tendons attach, such as the heel. A characteristic triad can occur: arthritis, eye inflammation (conjunctivitis or uveitis) and urinary or genital inflammation. There may also be skin and mouth changes. Blood tests show inflammation but no specific diagnostic marker; HLA-B27 supports the diagnosis and is linked to a more prolonged course. Most cases improve over three to six months with anti-inflammatory treatment, though some become chronic or recur.

• ESR and CRP — usually raised, reflecting active inflammation.
• HLA-B27 — often positive; supports the diagnosis and predicts a more prolonged or severe course.
• Tests for the triggering infection — stool, urine or genital swabs as appropriate.
• Rheumatoid factor and anti-CCP — typically negative, helping distinguish it from rheumatoid arthritis.
• Joint fluid analysis — to exclude infection or gout in a hot, swollen joint.

• Joint pain and swelling, often in the knees, ankles and feet, and usually asymmetric
• A whole swollen finger or toe (dactylitis)
• Heel pain or pain where tendons attach
• Red, sore or gritty eyes (conjunctivitis or uveitis)
• Pain or discomfort passing urine
• Low back or buttock pain
• Skin or mouth changes
• Often a preceding gut or genital infection 1–4 weeks earlier