Diagnosis by an experienced rheumatologist is the closest thing to a gold standard. 1).
The Assessment of SpondyloArthritis International Society (ASAS) presented its recommendations for a modified Berlin Algorithm 2) as a potentially useful tool for rheumatologists in diagnosing AS. SI joint involvement is the sine qua non for a definite diagnosis. Diagnosis is very involved, and includes: chronic low back pain, buttock pain, sacroiliitis, family history, psoriasis, inflammatory bowel disease or arthritis followed in ≤ 1 month with urethritis, cervicitis or acute diarrhea, an enthesopathy, and family history, and positive X-rays.
The (obsolete) New York Criteria was an early attempt to establish diagnostic benchmarks, but should no longer be used for definitive diagnosis.