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There is no one single test that is able to diagnose AS. Instead the diagnosis is derived from a combination of clinical and radiological features. This has led to a number of criteria that have aimed to be most specific in identifying those affected with the disease. The most commonly used criteria were created in New York, which were modified in 1984. The criteria includes:
- Clinical criteria
- Low back pain and stiffness for more than 3 months which improves with exercise, but is not relieved by rest
- Limitation of motion of the lumbar spine in both the sagittal (front-back) and frontal planes (side-side) – which refers to the direction of movement
- Limitation of chest expansion relative to normal values corrected for age and sex – which reflects the degree of movement in the midback where it joins onto the rib cage.
- Radiologic criteria
- Sacroiliitis on radiographs
Definite AS is diagnosed if the radiologic criteria is satisfied with the presence of at least 1 clinical criterion
Probable AS is diagnosed if;
- Three clinical criteria are present
- Radiologic criterion present without any signs or symptoms satisfying the clinical criteria
The main aim of these diagnostic criteria, such as the modified New York criteria, is to ensure a uniform population of patients with AS for the purpose of research. Consequently, such an approach to diagnosis is unlikely to identify patients with early disease because of the need to have limitation of motion and the presence of radiological change, both of which often requires a significant amount of time to develop. Therefore, recent interest has focused on how to identify patients in the early phase of their illness.
In this regard the Assessment of SpondyloArthritis international Society (ASAS) developed criteria for the diagnosis of Axial Spondyloarthritis, which defines the scenario when an inflammatory spinal disease is present that does not fulfill the criteria for a specific subtype. The criteria is presented below;
In this criterion, imaging includes the use of MRI scanning, which is able to demonstrate inflammation at the affected joints. The advantage of this approach is that patients can be identified early, prior to the onset of any structural damage, such that treatment can be commenced with the aim of retarding or ideally preventing the long-term sequelae of the disorder. This is further emphasized by the fact that a large proportion of disease progression and functional loss occurs in the first ten years of the disease.
There are a number of other investigations that are useful for the Rheumatologist in the assessment of Ankylosing Spondylitis and the related disorders, even though these may not be necessary or useful in the diagnosis. Of these, the inflammatory markers are worthy of mention. The markers of inflammation that are most commonly measured are the C-Reactive Protein (CRP) and the Erythrocyte Sedimentation Rate (ESR). These measures are elevated at some time in the course of the disease in 50-70% of patients, which may but not always correlate with disease activity. As such, these tests, may have a limited role in the diagnosis and monitoring of the disease. It seems that these tests are most useful in those patients who have an arthritis affecting joints other than the spine. In addition, ultrasound evaluation, especially with Power Doppler, has proven useful when assessing for inflammation at the sites where tendons join onto bone, called entheses, especially when that inflammation is not clinically apparent. However, ultrasound has not been of much use in the evaluation of spinal symptoms.