Ankylosing spondylitis (AS) is a systemic disease of unknown origin with chronic inflammation of the medial joints, the incidence of which is about 0.3%. The main features of AS are the involvement of the sacroiliac joint in almost all cases, resulting in invasive aseptic inflammation of the medial bone and its periarticular tissues, including the sacroiliac joint, ligamentous calcification and bony ankylosis. AS has an insidious onset, a long course, and a variety of onset, making it difficult to diagnose. Some patients are referred to various clinical departments and are easily diagnosed as other connective tissue diseases when there is multisystem involvement. The misdiagnosis rate has been reported in the literature to be 65%-76% [2]. The misdiagnosed diseases are rheumatoid arthritis, rheumatoid arthritis, gout, Ritter’s syndrome, and lumbar disc herniation, in that order [3]. Due to the high rate of misdiagnosis, the treatment of this disease is particularly difficult because it is generally limited to the level of non-steroidal anti-inflammatory drugs or the wrong long-term use of high doses of hormones [4]. In order to reduce the misdiagnosis and improve the diagnosis rate, it is necessary to make an analysis and summary of the misdiagnosis of this disease. 1. The danger of misdiagnosis AS has a slow progression, with a minimum of 5 months and a maximum of 30 years, with an average of 5.5 years. Clinical data show that even with the application of advanced technologies such as CT and MRI, the average duration of the disease has reached about 6-7 years at the time of diagnosis [5]. It is characterized by almost total involvement of the sacroiliac joint, with upward spread of the lesion along the spine, chronic inflammation and erosion at metabolically active bone attachment points such as tendons and ligaments, resulting in granulation tissue formation, calcification of the affected area, and new bone formation. Patients tend to experience calcification of the intervertebral disc annulus and adjacent ligaments, which over time results in complete ossification of the entire spinal ligaments and the formation of bony bridges, resulting in ankylosis of the spinal joints. In this case, a bamboo-like spine or a hunchback deformity may appear, and breathing may be affected by the ankylosis of the cribriform joints. Since the disease cannot be reversed, the disability rate is very high, which brings great pain to the life and work of patients and a heavy burden to society and families. Many patients are already in the middle or late stages of the disease when it is detected. There is no specific test to confirm the diagnosis of AS, and it is difficult to make early diagnosis. Only 6.3% of patients are diagnosed at the time of initial presentation, and only 43.2% are diagnosed within 3 years, and a few patients are diagnosed only after 15 years. By the time of admission, 43.0% of patients have varying degrees of disability [6], and some patients even require total hip replacement [7] and pulmonary fibrosis. The key to reducing the disability rate of AS is early diagnosis and proper treatment. The immediate danger of misdiagnosis is that it leads to mistreatment. Take misdiagnosis of lumbar disc herniation as an example, patients with lumbar disc herniation need bed rest during the acute phase, while AS requires patients to exercise more to prevent joint stiffness and stiffness in order to maintain joint motion and prevent disability with motion. If a patient with AS is misdiagnosed as having lumbar disc herniation and is required to rest in bed and reduce activity, the patient’s spine can become irreversibly straightened over time. The current treatment of AS is basically symptomatic and generally remains at the level of NSAIDs. Even the use of salazosulfapyridine is only effective in early stages or in the presence of peripheral arthropathy, but is less effective in those who are already ankylosed. There is no cure for AS because the cause is not known [8]. Therefore, the basis for reducing the disability rate of AS is to reduce misdiagnosis and confirm the diagnosis early. 2. Reasons for misdiagnosis 2.1 Poor concept of AS: The reason for misdiagnosis is that some specialists have old concepts, and many patients with AS are wrongly diagnosed and treated as rheumatoid arthritis. Some physicians still consider AS as a central type of rheumatoid and still misdiagnose it in the face of typical cases [6,9]. Even when patients present with low back stiffness and spinal ankylosis AS is still not considered, reflecting the lack of basic understanding of the clinical features of the disease by some clinicians. Many radiologists are unfamiliar with the clinical significance of the disease and sacroiliac joint changes, even though patients have obvious pathological changes such as gross, blurred, worm-like or cystic changes on the sacroiliac joint surface, narrowing or even fusion of the joint space, they still mistake them for normal or osteophytes. One of the main reasons for misdiagnosis is not paying attention to the importance of consultation, not understanding the characteristics of the disease, and not being alert to AS, which has the characteristics of morning stiffness and aggravation of lumbosacral pain or stiffness after rest, especially after long sleep, and relief after activity. In contrast, low back pain due to strain or injury diseases, such as lumbar disc disease, is aggravated after activity and improves after rest [10]. We cannot analyze the cases comprehensively and diagnose them blindly, diagnosing “rheumatism and disc prolapse” for lumbar pain, “joint tuberculosis and femoral head necrosis” for hip pain, “sciatica” for lower limb and hip pain, etc. [10]. sciatica”, etc. [11]. In addition, treating physicians rarely examine and record the back and lumbosacral region of patients with lumbosacral pain for lumbar flexion and extension and lateral bending, and thoracic dilatation, and thus fail to detect AS [12]. In particular, some physicians do not understand that sacroiliac arthritis is a characteristic change of AS and do not examine the sacroiliac joints carefully for pain, which results in missed diagnosis. 2. 2 Neglect of peripheral joint manifestations of AS: The current common diagnostic criteria for AS include only spinal symptoms, and the term ankylosing spondylitis has led to the neglect of peripheral joint and extra-articular lesions in AS, causing many clinicians to interpret AS as a simple spinal lesion, which is one of the reasons for the high rate of misdiagnosis and underdiagnosis of AS. Many patients with AS have peripheral joint pain as their first symptom, such as ankle pain or heel pain, and are often misdiagnosed as rheumatic or rheumatoid. According to statistics, 72.6% of patients with peripheral joint involvement and 29.9% of patients with extra-articular systemic damage have aseptic inflammation of tendon or ligament attachment points as the first symptom, and the younger the patient is, the more obvious this feature is, manifesting as asymmetric hip and knee joints or single joint swelling and pain, mostly accompanied by hypothermia and excessive sweating, and easily misdiagnosed as joint tuberculosis, rheumatoid arthritis or rheumatoid arthritis [13]. The rate of misdiagnosis is as high as 69.4% for those with extra-articular damage and peripheral joint involvement and 25.2% for those without extra-articular damage, with a significant difference between the two [2]. The onset of AS in children is more common than expected, and almost all of them have sacroiliac joint x-ray changes and peripheral joint involvement, with 75% of them presenting with this first symptom [14,15]. The earlier the age of onset of AS in children, the more likely it is to be misdiagnosed [16]. 2.3 Over-reliance on or neglect of ancillary examinations: because some clinicians lack knowledge of the clinical features of AS or are satisfied with X-ray examinations only, they over-rely on imaging examinations of the sacroiliac joints and neglect the clinical manifestations and physical examinations of AS, thus easily causing misdiagnosis and omission [17]. On the other hand, due to the complexity of pelvic structures and sacroiliac joints, it is difficult for radiologists to judge the grade I-II joint changes, resulting in early diagnosis of some AS patients and misdiagnosis. In addition, they are not familiar with the purpose and significance of the relevant laboratory tests, and are not clear about the specific clinical significance of the five rheumatic tests. The diagnosis of rheumatic fever or rheumatism is made when anti-O is found to be elevated; patients with elevated ESR or CRP are not alerted and are still diagnosed with lumbar muscle strain or lumbar disc herniation. etc.; denying the diagnosis of AS based on HLA-B27 negativity. 3. Measures to avoid misdiagnosis 3.1 Strengthening education: Given that there are still a considerable number of clinicians and radiologists who are not familiar with this disease, relevant societies and hospital business management should organize relevant personnel to strengthen their study in order to understand AS as a disease and to be familiar with and master the basic features and clinical diagnostic or screening criteria of AS. In addition, an assessment system should be implemented to urge physicians to strengthen continuing education and learning in order to promote their continuous mastery and understanding of new advances, knowledge and concepts in the discipline. The misdiagnosis of AS can be greatly reduced as long as the physicians of related disciplines understand and pay attention to AS. 3.2 Emphasis on the importance of physical examination: Many misdiagnoses can be avoided by paying attention to the most basic visual, tactile, and percussion examinations. The initial diagnosis can be made by the patient’s facial color, bending (pointing distance), plank waist, sacroiliac joint examination (4-character test and percussion pain, etc.). 40 years old, insidious occurrence with morning stiffness, chronic low back pain relieved by activity are the characteristic symptoms of AS, so pay attention to these characteristics during the consultation. Since sacroiliac arthritis is the hallmark feature of AS, special examination of the sacroiliac joint is the focus of the physical examination. Pain at the bone attachment points of individual joints, tendons and ligaments of the lower limbs should be taken seriously. The key to diagnosis is the establishment of radiological sacroiliac arthritis [18]. X-ray plain film is convenient and easy to use and is the first choice for the diagnosis of AS. Orthopantomogram of the pelvis should be routinely performed in patients suspected of having AS. ESR and CRP are the simplest and most economical tests to reflect disease activity, and they are elevated in about 50%-70% of patients during the active phase, especially CRP, which can be used to differentiate strain disease. In contrast, HLA-B27 has limited diagnostic significance. Early diagnosis of AS is currently unrealistic, and it is a process from atypical to typical, from no bone destruction to typical x-ray sacroiliac joint manifestations. None of the diagnostic criteria for AS to date are suitable for the early diagnosis of the disease [20]. Lack of awareness of AS features and failure to perform appropriate physical examination and sacroiliac joint X-ray often leads to misdiagnosis, while primary care hospitals are more prone to misdiagnosis. In cases of lumbosacral pain with morning stiffness or swelling or pain in the joints of the lower limbs and at the attachment points of tendon ligaments, careful inquiries should be made about whether there is a family history or a history of iritis. For patients suspected of AS, pelvic X-ray or CT film of sacroiliac joint should be taken and necessary laboratory tests should be performed to confirm the diagnosis and treatment as soon as possible. In addition, attention should be paid to the differential diagnosis of sacroiliac joint lesions, such as dense iliitis, sacroiliac joint infection, tuberculosis and tumor.