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ASAS Criteria: Diagnosing Axial Spondyloarthritis

The ASAS criteria are internationally recognised classification criteria used to diagnose axial spondyloarthritis, including ankylosing spondylitis.

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The ASAS criteria are internationally recognised classification criteria used to diagnose axial spondyloarthritis, including ankylosing spondylitis.

What are the ASAS Criteria?

The ASAS criteria (Assessment of SpondyloArthritis international Society) are internationally recognised classification criteria developed by the ASAS expert group and published in 2009. They are used to classify and identify patients with axial spondyloarthritis (axSpA), a chronic inflammatory rheumatic disease primarily affecting the spine and sacroiliac joints. The criteria help to detect the disease at an early stage, before irreversible structural damage to the spine occurs.

Background and Clinical Relevance

Axial spondyloarthritis encompasses two forms:

Radiographic axial spondyloarthritis (also known as ankylosing spondylitis or Bechterew disease): Structural changes are visible on X-ray.
Non-radiographic axial spondyloarthritis: No X-ray changes are detectable yet, but MRI findings and clinical features may already be present.

By covering both forms, the ASAS criteria allow earlier diagnosis and treatment initiation, which is essential to prevent long-term disability.

Structure of the ASAS Classification Criteria

The ASAS criteria apply to patients with chronic back pain lasting at least 3 months and onset before the age of 45. Classification can be achieved via two pathways:

1. Imaging Arm

Evidence of sacroiliitis on imaging (MRI or X-ray) plus at least 1 additional feature of spondyloarthritis from the following list:

Inflammatory back pain
Arthritis
Enthesitis (heel pain)
Uveitis (eye inflammation)
Dactylitis (sausage digit)
Psoriasis
Crohn disease or ulcerative colitis
Good response to non-steroidal anti-inflammatory drugs (NSAIDs)
Family history of spondyloarthritis
Positive HLA-B27
Elevated C-reactive protein (CRP)

2. Clinical Arm

Positive HLA-B27 plus at least 2 additional features of spondyloarthritis from the list above (without imaging evidence of sacroiliitis).

Key Features Explained

Inflammatory Back Pain

Inflammatory back pain is the cardinal symptom of axial spondyloarthritis. It typically begins before age 40, develops gradually, improves with exercise but not with rest, and is characterised by nocturnal pain and morning stiffness lasting more than 30 minutes.

HLA-B27

HLA-B27 is a genetic tissue marker (human leukocyte antigen) found in the majority of patients with axial spondyloarthritis. A positive result significantly increases the probability of the disease, but is not diagnostic on its own.

Sacroiliitis on Imaging

Sacroiliitis refers to inflammation of the sacroiliac joints. MRI can detect early active inflammatory changes (bone marrow oedema), while X-ray reveals structural damage such as erosions and sclerosis in later disease stages.

Diagnosis and Further Investigations

Although the ASAS criteria were primarily designed as classification criteria for research purposes, they also serve as a valuable diagnostic aid in clinical practice. A definitive diagnosis of axial spondyloarthritis requires comprehensive evaluation by a rheumatologist, including:

Detailed medical history and physical examination
Laboratory tests (HLA-B27, CRP, full blood count)
Imaging: MRI of the sacroiliac joints and/or X-rays of the spine

Treatment of Axial Spondyloarthritis

Early diagnosis using the ASAS criteria enables timely treatment. Therapeutic options include:

Physiotherapy and regular exercise: Essential for maintaining spinal mobility.
Non-steroidal anti-inflammatory drugs (NSAIDs): First-line treatment for pain relief and inflammation control.
Biologics (TNF inhibitors, IL-17 inhibitors): Used when NSAIDs are insufficient; they target specific inflammatory pathways.
JAK inhibitors: A newer drug class also approved for the treatment of axial spondyloarthritis.

References

Rudwaleit M et al. - The development of Assessment of SpondyloArthritis international Society classification criteria for axial spondyloarthritis. Annals of the Rheumatic Diseases, 2009; 68(6): 770-776.
Sieper J, Poddubnyy D - Axial spondyloarthritis. The Lancet, 2017; 390(10089): 73-84.
van der Linden S, van der Heijde D - Ankylosing spondylitis. In: Kelley's Textbook of Rheumatology, 9th edition. Elsevier, 2013.

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