Classification Criteria for Diagnosis

Ankylosing spondylitis (AS) can be challenging to diagnose, and many people with AS experience symptoms for several years before receiving a correct diagnosis. To aid doctors in making a proper diagnosis, classification criteria have been developed by different groups. These diagnostic criteria aim to lessen the time between the onset of symptoms and a proper diagnosis and beginning treatment. Because AS generally begins at a young age, with symptoms starting in late adolescence or early adulthood, it can greatly impact a person during a time of their life that is usually very active and productive. Earlier diagnosis and treatment can lessen the impact of AS on an individual’s disease burden and improve their quality of life.¹

ASAS classification criteria for spondyloarthritis

Spondyloarthritis (SpA) is a general term that encompasses a group of inflammatory arthritis conditions. The primary form of SpA is AS. Some of the common features of SpA are an association with the genetic marker HLA-B27, a pattern of joint inflammation that is asymmetric (not on both sides of the body), and possible inflammation of the points where ligaments and tendons attach to the bones (enthesitis).¹

Diagnostic criteria for SpA have been developed by the Assessment of SpondyloArthritis International Society (ASAS). The ASAS criteria classifies SpA as axial SpA (affecting the spine) or peripheral SpA (affecting joints in the arms or legs). Axial SpA can also be classified as radiographic or non-radiographic. Radiographic SpA means that inflammation and/or joint damage is visible on imaging, such as x-ray or magnetic resonance imaging (MRI). The term radiographic SpA is also used in place of AS, and these terms refer to the same condition. Non-radiographic means that while the person’s joints appear normal on imaging, they have other symptoms of SpA.^1-3

The criteria require that a person has had back pain for at least 3 months and age of symptoms beginning at less than 45 years and one of the following:

• Inflammation of the sacroiliac joints seen on imagine plus at least one SpA feature
• HLA-B27 marker plus at least two SpA features

Axial SpA can be diagnosed using the ASAS criteria if the individual has inflammation in the sacroiliac joints plus at least one other SpA feature, or if the person has the HLA-B27 marker and at least two other SpA features. SpA features include¹:

• Inflammatory back pain
• Arthritis
• Enthesitis (heel)
• Uveitis (inflammation in the eyes)
• Dactylitis (inflammation in the fingers or toes that causes a sausage-like appearance)
• Psoriasis
• Inflammatory bowel disease (such as Crohn’s disease or ulcerative colitis)
• A good response to NSAIDs (non-steroidal anti-inflammatory drugs)
• A family history of SpA
• HLA-B27 marker
• Elevated C-reactive protein (CRP)

Modified New York criteria

The modified New York criteria was developed in 1984 and has historically been used in clinical studies. However, critics have noted that the criteria are not applicable in diagnosing AS in its earliest stages (before sacroiliitis is visible on imaging).³

In the modified New York criteria, there are clinical criteria and radiological criteria. Radiological criteria are the appearance of the sacroiliac joints on x-rays, which are given a grading of 0 to 4. Grade 0 is normal, grade 1 is suspicious changes, grade 2 is minimal definite changes, grade 3 is distinctive changes, and grade 4 is ankylosis (fusion of the bones). To meet the criteria for a diagnosis of AS, a person must have grade 2-4 sacroiliitis on both sides or grade 3-4 sacroiliitis on one side as well as at least one of the following clinical criteria⁴:

• Low back pain for at least 3 months, which is improved by exercise and not relieved by rest
• Limitation of the lumbar (low) spine in flexibility
• Limitation of chest expansion

A challenging diagnosis

A major challenge that delays proper treatment of AS and SpA has been the difficulties in getting a prompt diagnosis. The criteria that have been developed can help doctors diagnose these diseases at an early stage, in hopes that effective treatment can begin and stop or delay the joint damage that can reduce mobility and quality of life.