ASAS Criteria for Axial Spondyloarthritis (SpA)

What are the ASAS Criteria?

The Assessment of SpondyloArthritis international Society (ASAS) criteria are a set of standardized classification criteria used to identify patients with axial spondyloarthritis (axSpA). These criteria were developed to improve early diagnosis and classification of spondyloarthritis, particularly in patients with chronic back pain.

The ASAS criteria include both clinical features and imaging findings, allowing for classification through either an imaging arm or a clinical arm, depending on the available diagnostic information.

Clinical Significance

Axial spondyloarthritis is a chronic inflammatory disease primarily affecting the spine and sacroiliac joints. Early diagnosis is crucial for appropriate management and prevention of long-term complications such as spinal fusion and disability.

The ASAS criteria help standardize the classification of axSpA, facilitating both clinical practice and research. They are particularly valuable for identifying patients with non-radiographic axial spondyloarthritis, a form of the disease that may not show definitive changes on conventional radiographs.

Classification Pathways

Imaging Arm:

A patient with chronic back pain (≥3 months) with age of onset < 45 years can be classified as having axial spondyloarthritis if they have:

• Sacroiliitis on imaging (MRI or X-ray) plus ≥1 SpA feature

Clinical Arm:

Alternatively, a patient can be classified as having axial spondyloarthritis if they have:

• HLA-B27 positive plus ≥2 other SpA features

SpA Features:

• Inflammatory back pain
• Arthritis
• Enthesitis (heel)
• Uveitis
• Dactylitis
• Psoriasis
• Crohn's disease/ulcerative colitis
• Good response to NSAIDs
• Family history of SpA
• HLA-B27 positive
• Elevated CRP

Interpretation

The ASAS criteria have a sensitivity of approximately 82% and a specificity of 88% for the classification of axial spondyloarthritis. They are primarily intended for classification in research settings but are also widely used to support clinical diagnosis.

It's important to note that these are classification criteria, not diagnostic criteria. Clinical judgment remains essential in the diagnosis of axial spondyloarthritis, particularly in atypical presentations.

Limitations

• The criteria are designed for patients with chronic back pain starting before age 45
• They may not be applicable to patients with predominantly peripheral symptoms
• Access to MRI and HLA-B27 testing may be limited in some settings
• Interpretation of sacroiliitis on imaging requires expertise
• The criteria do not account for all possible manifestations of spondyloarthritis