The best clues to the diagnosis of this disease are the patient’s symptoms, joint signs and extra-articular manifestations and family history.The most common and characteristic early complaint of AS is stiffness and pain in the lower low back. Since low back pain is an extremely common symptom in the general population, but most of it is mechanical non-inflammatory back pain, whereas this disease is inflammatory in nature. The following 5 items help to differentiate inflammatory back pain caused by spondylitis from non-inflammatory back pain caused by other causes I. Physical examination Sacroiliac joint and paravertebral muscle pressure is a positive sign in the early stages of the disease. As the disease progresses, the lumbar lordosis flattens, the movement of the spine is restricted in all directions, the extension of the thorax is reduced, and the cervical vertebrae protrude. The following methods can be used to check the progression of sacroiliac joint pain or spinal lesions: 1. Occipital wall test In a normal person, when the heels are pressed against the wall in an upright position, the back of the occiput should be close to the wall without a gap. In the case of cervical stiffness and/or thoracic segmental deformity, the gap increases to more than a few centimeters, resulting in the occipital area not being able to fit against the wall. The normal value of the difference between deep inspiration and deep expiration is not less than 2.5 cm, while the expansion of the thorax is reduced in those with extensive rib and spinal involvement. 3.Schober test Mark the vertical distance of 10cm above and 5cm below the midpoint of the posterior superior iliac spine line, and then ask the patient to bend over (keep both knees in upright position) to measure the maximum forward flexion of the spine. 4.Pelvic pressure The patient lies on his side, and pressure on the pelvis from the other side can cause pain in the sacroiliac joint. The patient is lying on his side and pressing the pelvis from the other side can cause sacroiliac joint pain. 5.Patrick test (lower extremity 4-character test) The patient lies on his back with one knee flexed and the heel placed on the opposite knee that is straight. The examiner presses the flexed knee with one hand (when the hip is in flexion, abduction and external rotation) and presses the opposite pelvis with the other hand, and the pain of the opposite sacroiliac joint can be induced. Those with knee or hip lesions cannot complete the 4-character test either. The earliest change of AS occurs in the sacroiliac joint. X-rays of this area show blurring of the subchondral bone margin, bone erosion, blurring of the joint space, increased bone density and joint fusion. The degree of lesion of sacroiliac arthritis on X-ray is usually classified into 5 grades: grade 0 is normal, grade I is suspicious, grade II has mild sacroiliac arthritis, grade III has moderate sacroiliac arthritis, and grade IV has joint fusion ankylosis. Computed tomography (CT) should be used in clinically suspicious cases where the X-ray has not yet shown clear or grade II or higher bilateral sacroiliac arthritic changes. This technique also has the advantage of having fewer false positives. However, because the upper part of the sacroiliac joint anatomy is ligamentous, the irregularity and widening of the joint space on imaging caused by its attachment makes the judgment difficult. In addition, subchondral aging of the iliac portion of the sacroiliac joint similar to joint space narrowing and erosion is a natural phenomenon and should not be considered abnormal. Magnetic resonance imaging (MRI) is better than CT for understanding cartilage lesions, but it is prone to false positive results in determining sacroiliac arthritis, and because it is expensive, it should not be done as a routine examination at present. Radiographs of the spine show vertebral osteoporosis and square changes, blurring of the vertebral tuberosities, calcification of the paravertebral ligaments, and bone bridge formation. Extensive and severe ossifying bridges in advanced stages are called “bamboo-like spine”. Bone erosion at the pubic symphysis, sciatic tuberosity, and tendon attachment points (e.g., heel bone), with reactive sclerosis and villous changes in adjacent bone, may result in new bone formation. Laboratory tests: In active patients, increased blood sedimentation, increased C~reactive protein and mild anemia are seen. Rheumatoid factor is negative and immunoglobulins are mildly elevated. Although the rate of HLA-B27 positivity in AS patients is about 90%, there is no diagnostic specificity because normal people also have HLA-B27 positivity, and HLA-B27 negative patients cannot be excluded from AS as long as their clinical manifestations and imaging examinations meet the diagnostic criteria. IV. Diagnostic criteria Different criteria have been used in recent years, but the 1966 New York criteria, or the 1984 revised New York criteria, are still used. However, for those who temporarily do not meet the above criteria, reference can be made to the European preliminary diagnostic criteria for spondyloarthropathies, and those who meet them can also be included in this category for diagnosis and treatment, and follow-up observation. 1.New York criteria (1966): Bilateral or unilateral sacroiliac arthritis confirmed by X-ray (according to the aforementioned grade 0~IV) with 1 or 2 of the following clinical manifestations, respectively, AS requires: bilateral sacroiliac arthritis of grade III~IV confirmed by X-ray with at least 1 of the above clinical manifestations; or unilateral sacroiliac arthritis of grade III~IV or bilateral sacroiliac arthritis of grade II confirmed by X-ray, with 1 or 2 of the above clinical manifestations, respectively. 2.Revised New York criteria (1984), the duration of lower back pain lasts at least 3 months, the pain improves with activity but does not decrease with rest; the lumbar spine is restricted in anterior-posterior and lateral flexion; thoracic extension is less than normal for the same age and sex; bilateral sacroiliac arthritis grade II-IV, or unilateral sacroiliac arthritis grade III-IV. 3, European Spondyloarthropathy Study Group criteria: inflammatory spinal pain or asymmetric synovitis with predominantly lower extremity joints, with any of the following additional items