Should I Order This Test?

Could This Be Axial Spondyloarthritis?

Identify patients with chronic back pain who may have AxSpA and determine whether HLA-B27 testing, MRI sacroiliac joints, or rheumatology referral is warranted. AxSpA is frequently diagnosed 5–10 years late because inflammatory features are missed in primary care.

📋 ASAS Classification Criteria · ASAS Referral Strategy · Rudwaleit et al. IBP Criteria
This tool applies only to inflammatory back pain — not all back pain. Complete Step 1 first. If the screening criteria are not met, the calculator will indicate that AxSpA is unlikely regardless of other features.
1
Screening Criteria
Both must be present to apply this tool. Derived from the ASAS referral strategy — sensitivity ~85–90% for AxSpA.
⚠ Both required to proceed
Chronic back pain >3 months
Persistent, not episodic or acute
Age at onset <45 years
AxSpA almost always begins before age 45 — late onset should prompt other diagnoses
⛔ AxSpA Unlikely by Screening Criteria
Both criteria must be present. Back pain onset after age 45 or duration under 3 months makes AxSpA significantly less likely. Consider mechanical, degenerative, or other causes. If both criteria are present, check both boxes above to proceed.
2
Inflammatory Back Pain Features
ASAS inflammatory back pain criteria. Select all present. Meeting ≥4 of 5 has sensitivity ~77%, specificity ~72%.
Morning stiffness >30 minutes
Sensitivity 76% · Specificity 59%
+1
Improves with exercise, not rest
Sensitivity 82% · Specificity 54% — key distinguishing feature from mechanical pain
+1
No improvement with rest
Sensitivity 73% · Specificity 58%
+1
Night pain — wakes in second half of night
Sensitivity 64% · Specificity 62% — must get up and move to relieve it
+1
Alternating buttock pain
Sensitivity 37% · Specificity 85% — highly specific for sacroiliitis when present
+1
0
of 5 IBP features
≥4 features = inflammatory back pain

Mechanical back pain worsens with activity and improves with rest — the opposite pattern. The combination of night pain + improvement with exercise is highly suggestive of an inflammatory etiology.

3
Additional SpA Features
Each feature increases the likelihood of AxSpA. Include lab or imaging results if already available.
HLA-B27 positive
LR+ ≈ 9 — single strongest individual predictor of AxSpA
+3
Sacroiliitis on MRI
LR+ ≈ 9–10 — detects active inflammation years before X-ray changes
+3
Sacroiliitis on X-ray
LR+ ≈ 7–8 — indicates radiographic AxSpA (ankylosing spondylitis)
+3
History of acute anterior uveitis
LR+ ≈ 7 — painful red eye, photophobia, often recurrent; confirmed by ophthalmology
+2
Psoriasis (current or past)
LR+ ≈ 2–3 — psoriatic SpA has significant axial overlap
+2
Inflammatory bowel disease (Crohn's or UC)
LR+ ≈ 3–4 — up to 10–20% of IBD patients develop AxSpA
+2
Family history of SpA
LR+ ≈ 2–3 — first-degree relative with AS, AxSpA, psoriasis, IBD, or uveitis
+1
Elevated CRP (unexplained)
LR+ ≈ 2–3 — note: up to 50% of AxSpA patients have normal CRP
+1
i
ASAS Classification Pathways
Two routes to formal AxSpA classification — either imaging or clinical evidence plus SpA features.
Imaging Arm
Sacroiliitis on MRI or X-ray
+ ≥1 SpA feature
Sensitivity: ~82–84%
Specificity: ~84–87%
Clinical Arm
HLA-B27 positive
+ ≥2 SpA features
Sensitivity: ~82%
Specificity: ~84%

MRI can detect sacroiliitis years before radiographic changes appear. Among referred patients with inflammatory back pain, AxSpA prevalence is 20–40%. Among all chronic back pain under 45, prevalence is ~5%.

Clinical Pearls — Features Often Missed by PCPs
📅
Age of Onset
AxSpA almost always begins before age 45. Late-onset inflammatory back pain should prompt other diagnoses.
🏃
Exercise Improves Pain
Unlike mechanical pain, inflammatory back pain improves with activity — patients often feel worse after prolonged rest.
🌙
Night Pain
Patients wake in the second half of the night and must get up and move to relieve symptoms.
↔️
Alternating Buttock Pain
Pain alternating between buttocks is highly specific (85%) for sacroiliitis — don't miss this on history.
📋 ASAS Fast-Track Referral Rule
This patient meets the ASAS referral strategy: chronic back pain >3 months + onset <45 + at least one SpA feature (HLA-B27, IBP, uveitis, psoriasis, or IBD). Sensitivity ~85–90% for identifying AxSpA. Referral to rheumatology is recommended regardless of total score.
Suspicion Score
0
pts
0–2 Mech. 3–5 Possible ≥6 High
Score Breakdown
IBP features —
IBP score pts—
SpA features—
Recommendation
🔩
Complete the screening criteria to begin.
⛔ Not Eligible
Screening criteria not met
Both criteria must be present: back pain >3 months and onset before age 45. AxSpA is unlikely if either is absent.
🟢 Mechanical Likely
Inflammatory back pain features absent
Low probability of AxSpA (<5%). Features suggest mechanical or degenerative etiology. No rheumatologic testing indicated at this stage. Conservative therapy appropriate.
🟡 Possible AxSpA
Inflammatory features present
Probability AxSpA ~10–25%. Consider initial testing while monitoring symptoms.
Consider Ordering
HLA-B27 CRP / ESR CBC X-ray sacroiliac joints
🔴 High Suspicion AxSpA
Referral recommended
Probability AxSpA >40–50%. MRI sacroiliac joints is the preferred imaging — detects active sacroiliitis years before X-ray. Early diagnosis prevents structural damage.
Recommended Next Steps
Rheumatology referral MRI sacroiliac joints HLA-B27 CRP / ESR CBC / CMP
Scoring Reference
Inflammatory Back Pain (max 5)
Morning stiffness >30 min+1
Improves with exercise+1
No improvement with rest+1
Night pain+1
Alternating buttock pain+1
SpA Features
HLA-B27 positive+3
Sacroiliitis on MRI+3
Sacroiliitis on X-ray+3
Uveitis history+2
Psoriasis+2
IBD+2
Family history SpA+1
Elevated CRP+1
0–2 Mechanical  ·  3–5 Possible  ·  ≥6 High suspicion

Frequently Asked Questions

Common questions clinicians and patients ask after using this tool — and after receiving these results.

What is the difference between axial SpA and ankylosing spondylitis? ▼
Ankylosing spondylitis (AS) is the older term for axial spondyloarthritis (axial SpA) where structural damage is visible on X-ray — specifically sacroiliitis seen on plain radiograph. Non-radiographic axial SpA (nr-axSpA) refers to the same disease process but without X-ray changes, typically detected by MRI showing active sacroiliac joint inflammation. Both conditions cause the same symptoms, have the same genetic risk factors (HLA-B27), and respond to the same treatments. The distinction matters mainly because it reflects how early in the disease course the patient is being evaluated.
What does HLA-B27 positive mean? ▼
HLA-B27 is a genetic marker (a type of human leukocyte antigen) present in approximately 6–8% of the general population. Its significance depends entirely on clinical context. HLA-B27 is positive in approximately 85–90% of patients with axial SpA, making it a useful diagnostic tool when inflammatory back pain is present. However, a positive HLA-B27 alone is not a diagnosis — 95% of HLA-B27 positive individuals never develop axial SpA. It is best interpreted alongside the full clinical picture using tools like this calculator.
How is axial SpA different from mechanical back pain? ▼
Inflammatory back pain — the type caused by axial SpA — has a distinct pattern: onset before age 45, insidious onset, improvement with exercise, no improvement with rest, nighttime waking in the second half of the night, and morning stiffness lasting more than 30 minutes. Mechanical back pain typically worsens with activity and improves with rest. In practice, inflammatory back pain is often misdiagnosed as mechanical back pain for years. The average diagnostic delay in axial SpA is still 7–10 years from symptom onset, and early treatment prevents structural damage.
Can women get axial SpA? ▼
Yes — axial SpA affects men and women roughly equally, though this was not recognized historically. The disease was long considered male-predominant because women tend to have less X-ray damage (less radiographic progression), leading to underdiagnosis. Women often present with predominantly peripheral arthritis, enthesitis, or isolated IBP without classic bamboo spine changes. HLA-B27 positivity rates are similar in men and women. Women with axial SpA are significantly underdiagnosed and undertreated compared to men, and this gap remains an active area of clinical concern.
What is the ASAS referral rule and why does it matter? ▼
The ASAS (Assessment of SpondyloArthritis International Society) fast-track referral criteria state that a patient with chronic back pain (≥3 months) and onset before age 45 should be referred to rheumatology if they have a positive HLA-B27 OR sacroiliitis on imaging (MRI or X-ray). The rule was developed to reduce the 7–10 year average diagnostic delay in axial SpA. The sensitivity of this rule is approximately 79% with a specificity of 83%. This calculator incorporates these criteria to flag patients who meet the referral threshold.
What joints besides the spine are affected in axial SpA? ▼
Axial SpA can cause peripheral joint involvement in approximately 30–40% of patients, most commonly affecting the hips, shoulders, knees, and ankles. Enthesitis (inflammation where tendons and ligaments attach to bone) is characteristic — commonly at the Achilles tendon, plantar fascia, and around the pelvis. Uveitis (eye inflammation causing painful red eye and photophobia) occurs in ~25% of patients. Associated conditions include inflammatory bowel disease (Crohn's, UC) and psoriasis. These extra-articular features can be the presenting complaint.
References
  1. Rudwaleit M, et al. Inflammatory back pain in ankylosing spondylitis: a reassessment of the clinical history for application as classification and diagnostic criteria. Arthritis Rheum. 2006;54(2):569–578.
  2. Rudwaleit M, et al. The development of Assessment of SpondyloArthritis international Society classification criteria for axial spondyloarthritis. Ann Rheum Dis. 2009;68(6):777–783.
  3. Sieper J, et al. Axial spondyloarthritis. Lancet. 2017;390(10089):73–84.
  4. Braun J, Sieper J. Ankylosing spondylitis. Lancet. 2007;369(9570):1379–1390.
  5. van den Berg R, et al. Evaluation of the ASAS classification criteria for axial spondyloarthritis. Ann Rheum Dis. 2013;72(8):1342–1348.