The American College of Rheumatology published diagnostic guidelines for ReA in 1999, citing three major criteria:
- Characteristic findings on physical exam of the musculoskeletal system: asymmetrical oligo- or monoarthritis involving lower extremities. There may be enthesitis, dactylitis, or inflammatory back pain.
- Evidence of a preexisting extra-articular infection, either enteritis or urethritis. This is usually a history of acute diarrhea (enteritis) or urethritis 3 days to 6 weeks prior to the onset of ReA symptoms. Sometimes there is a positive culture of the inciting organism, but not always, as the symptoms may develop after the acute infection has resolved. If there is still diarrhea, culture the stool for Campylobacter, Salmonella, Shigella and Yersinia.
- No evidence for another cause of the arthritis. If there is fluid in the joint, it should be aspirated and cultured to exclude infection (septic arthritis). Other causes of arthritis include rheumatoid arthritis and crystal-induced arthritis, among others. Supporting laboratory tests can include elevated inflammatory markers in the blood, such as ESR and CRP.