The diagnosis of osteoarthritis can usually be made based on the patient’s complaints and the physician’s physical examination, but sometimes some relevant ancillary tests are still needed. (1) For patients with fever and polyarticular pain, routine blood tests, blood sedimentation and C-reactive protein tests should be performed to exclude rheumatism, rheumatoid arthritis and infectious arthritis. Patients with osteoarthritis have no abnormal changes in routine blood tests, but those with acute synovitis may show mild abnormalities. (2) Imaging ① X-ray: X-ray of the joints can record the abnormalities of the lesions in general, reflecting the degree of joint damage, the extent of lesion progression and the response to treatment. The diagnosis of osteoarthritis in the small joints of the spine and the sacroiliac joints is of high value. (3) Magnetic resonance imaging (MRI): MRI can clearly show bone, articular cartilage, meniscus, ligaments, synovial membrane and joint effusion, but it is usually not necessary. (3) Synovial fluid examination: Synovial fluid can detect blood, microorganisms and uric acid crystals in joints, which is of diagnostic value for traumatic arthritis, infectious arthritis and gouty arthritis, especially when it is difficult to diagnose some single arthritis, sometimes it is necessary to perform joint cavity aspiration for synovial fluid examination. (4) Arthroscopy and synovial biopsy. Arthroscopy can visualize the lesion and remove synovial tissue for pathological examination, and some treatments such as free body removal and synovectomy can be done under arthroscopy.