ASAS Criteria: Classification of Axial Spondyloarthritis (AxSpA).
Entry Criteria
SpA Features
Set Classification Factors
Enter the entry criteria, imaging/genetics status, and SpA features to check ASAS classification.
Guidelines & Evidence
Verified
Last Review: 2026-07-17
When to Use
When to Use
Classification of patients with suspected axial spondyloarthritis
Identifying patients with radiographic and non-radiographic axSpA
Standardising clinical trial enrolment for axSpA therapeutics
Earlier diagnosis compared to modified New York criteria, which require radiographic sacroiliitis
Guiding HLA-B27 and MRI testing in patients with chronic back pain
Patient Population
Adults (< 45 years) with chronic back pain (≥ 3 months) with onset before age 45. The ASAS criteria were designed to capture both radiographic (ankylosing spondylitis) and non-radiographic axial SpA, the latter defined by MRI or HLA-B27 positivity without definite X-ray changes.
Prerequisites
The patient must have chronic back pain with onset < 45 years of age AND either imaging evidence of sacroiliitis OR HLA-B27 positivity. The criteria are not intended for patients with back pain starting after age 45.
How it Works
Dual-Arm Structure
The ASAS criteria for axial SpA consist of two alternative arms. A patient meets classification if either arm is satisfied. Both arms require the entry criterion of chronic back pain onset before age 45.
Arm 1: Imaging Arm
Sacroliitis on imaging (X-ray or MRI) PLUS ≥ 1 SpA feature:
• Inflammatory back pain
• Arthritis
• Enthesitis (heel)
• Uveitis (anterior)
• Dactylitis
• Psoriasis
• Crohn's / ulcerative colitis
• Good response to NSAIDs
• Family history of SpA
• HLA-B27 positive
• Elevated CRP
Arm 2: HLA-B27 Arm
HLA-B27 positive PLUS ≥ 2 SpA features:
• Inflammatory back pain
• Arthritis
• Enthesitis (heel)
• Uveitis (anterior)
• Dactylitis
• Psoriasis
• Crohn's / ulcerative colitis
• Good response to NSAIDs
• Family history of SpA
• Elevated CRP
Peripheral SpA Criteria
For patients presenting with predominantly peripheral symptoms (arthritis, enthesitis, dactylitis) without axial symptoms, a separate ASAS peripheral SpA criteria set exists. It requires arthritis, enthesitis, or dactylitis plus ≥ 1 SpA feature (uveitis, psoriasis, IBD, preceding infection, HLA-B27, sacroiliitis on imaging) or ≥ 2 other features.
Clinical Pearls
Clinical Application
The ASAS criteria have dramatically improved the identification of early axSpA. Before 2009, the modified New York criteria required definite radiographic sacroiliitis (grade ≥ 2 bilaterally or grade 3–4 unilaterally), which could take 5–10 years to develop. By incorporating MRI and HLA-B27, the ASAS criteria enable classification years earlier. Patients with non-radiographic axSpA have similar symptom burden and quality of life impairment as those with radiographic AS.
Features Explained
Inflammatory back pain: defined by ASAS expert group as ≥ 4 of 5 features (age < 40, insidious onset, improvement with exercise, no improvement with rest, night pain)
Sacroliitis on MRI: active inflammatory lesions of SI joints (bone marrow oedema/osteitis) on STIR or T1 post-gadolinium sequences
Enthesitis: typically Achilles or plantar fascia insertion, confirmed by clinical examination or ultrasound
Good NSAID response: symptoms resolve within 24–48 hours of full-dose NSAID therapy
Pitfalls to Avoid
Do not apply the criteria to patients with back pain onset after age 45
MRI sacroiliitis can be seen in mechanical stress, pregnancy, and athletes; do not over-interpret
HLA-B27 is common in some populations (e.g., 6–8% of Caucasians); most HLA-B27+ people do not have SpA
The criteria are for classification, not diagnosis; clinical judgement remains essential
Elevated CRP alone is a weak SpA feature and should not be over-weighted
Next Steps
Classified as Axial SpA
01
Assess disease activity with BASDAI and ASDAS
02
Evaluate functional status with BASFI
03
Initiate NSAID therapy as first-line treatment
04
If inadequate response to NSAIDs, consider biologic therapy (anti-TNF, IL-17 inhibitor)
05
Screen for extra-articular manifestations: uveitis, psoriasis, IBD
06
Assess cardiovascular risk, as axSpA confers increased CVD risk
Not Classified
01
Consider alternative diagnoses (mechanical back pain, diffuse idiopathic skeletal hyperostosis, osteitis condensans ilii)
02
Re-evaluate in 6–12 months if symptoms persist
03
Repeat imaging if clinical suspicion remains high
04
Refer to rheumatology for expert evaluation if diagnostic uncertainty persists
The Evidence
Key Evidence
ASAS criteria demonstrated sensitivity of 82.9% and specificity of 84.4% in the original validation cohort
Enabled earlier classification of axSpA by a mean of 5–9 years compared to modified New York criteria
Non-radiographic axSpA patients show similar response to anti-TNF therapy as radiographic AS in randomised trials
ASAS criteria are endorsed by ACR, EULAR, and ASAS and are the global standard for axSpA classification
Primary Reference
The development of Assessment of SpondyloArthritis international Society classification criteria for axial spondyloarthritis (part II): validation and final selection.
Rudwaleit M et al. • Annals of the Rheumatic Diseases. 2009;68(6):777-83
The Assessment of SpondyloArthritis international Society classification criteria for peripheral spondyloarthritis and for spondyloarthritis in general.
Rudwaleit M et al. • Annals of the Rheumatic Diseases. 2011;70(1):25-31
Origins & History
Development
The ASAS classification criteria for axial spondyloarthritis were developed by the Assessment of SpondyloArthritis international Society (ASAS) and published in 2009. The criteria were derived from a large international cohort of patients with chronic back pain and were validated in a separate cohort. The development represented a paradigm shift by recognising that patients without radiographic sacroiliitis (non-radiographic axSpA) have a similar disease burden and treatment response as those with definite ankylosing spondylitis.
Last Comprehensive Review: 2026-07-17