Clinical Notes
What is axial spondyloarthritis? ▼
Axial SpA is a chronic inflammatory arthritis affecting the sacroiliac joints and spine. The ASAS criteria unified two subgroups: radiographic axSpA (ankylosing spondylitis) with definite X-ray sacroiliitis, and non-radiographic axSpA where radiographs are normal but MRI or clinical features are strongly consistent. Both forms respond to TNF inhibitors and IL-17 inhibitors.
What defines radiographic sacroiliitis? ▼
Modified New York criteria grading: Grade 0 = normal; Grade 1 = suspicious; Grade 2 = minimal (sclerosis, blurring); Grade 3 = moderate (erosions, widening/narrowing, partial ankylosis); Grade 4 = complete ankylosis. Radiographic sacroiliitis requires grade ≥2 bilaterally or grade 3-4 unilaterally. Interreader reliability for grade 2 is limited even among experienced readers.
What defines a positive MRI for sacroiliitis? ▼
An ASAS-positive MRI requires active inflammatory lesions - bone marrow edema (osteitis) in the subchondral bone on STIR or contrast-enhanced T1 fat-saturated sequences - that are highly suggestive of SpA. Structural lesions alone (erosions, fat metaplasia, sclerosis, ankylosis) without active inflammation are not sufficient. False positives occur in athletes, postpartum women, and other conditions.
What is inflammatory back pain (IBP)? ▼
ASAS 2009 IBP criteria require age at onset <40 years and at least 4 of 5 features: (1) insidious onset; (2) improvement with exercise; (3) no improvement with rest; (4) nocturnal pain improving upon getting up; (5) alternating buttock pain. Sensitivity 79.6%, specificity 72.4% vs. rheumatologist SpA diagnosis.
Imaging arm vs. clinical arm - when to use each? ▼
The imaging arm (specificity 97.3%) is preferred when sacroiliitis is confirmed on X-ray or MRI and represents about 60% of classified patients. The clinical arm (specificity 83.3%) was designed for when imaging is unavailable, requiring HLA-B27 plus 2 SpA features. Either arm is valid if criteria are met - patients may satisfy both.
How does HLA-B27 factor into the criteria? ▼
HLA-B27 serves two roles. In the clinical arm, it is the mandatory gating requirement. In the imaging arm, HLA-B27 positivity counts as one SpA feature. HLA-B27 is positive in approximately 80-90% of AS patients and 50-70% of nr-axSpA patients, but also in 5-8% of the general population without SpA.
What does "good NSAID response" mean as a SpA feature? ▼
Rapid and striking improvement in back pain within 24-48 hours of a full anti-inflammatory dose of an NSAID (naproxen 500mg BID, diclofenac 75mg BID, indomethacin 50mg TID), with recurrence within 48 hours of stopping. Partial or slow improvement does not qualify.
Can these criteria diagnose axSpA? ▼
No. These are classification criteria validated in patients referred to a rheumatologist with high clinical suspicion. In general populations, false-positive rates increase substantially. Clinical diagnosis requires full rheumatologic assessment integrating symptoms, examination, labs, and imaging.
Primary Source: Rudwaleit M, van der Heijde D, Landewe R, et al. The development of Assessment of SpondyloArthritis international Society classification criteria for axial spondyloarthritis (part II): validation and final selection.
Ann Rheum Dis. 2009;68(6):777-783.
doi:10.1136/ard.2009.108233
IBP Criteria: Sieper J, van der Heijde D, Landewe R, et al. New criteria for inflammatory back pain in patients with chronic back pain.
Ann Rheum Dis. 2009;68(6):784-788.