Pain Spa Clinical Update

Suspected Ankylosing Spondylitis: A Practical Work-Up, and Why MRI Now Comes First

Who to suspect, which tests to order, and when to refer

Dr Murli Krishna, Consultant in Pain Medicine, Pain Spa

Key points

• Think of axial spondyloarthritis (axSpA) in anyone with back pain lasting 3 months or more that began before the age of 45.
• No single symptom, examination finding, blood test or scan makes the diagnosis. It rests on the overall pattern.
• A normal CRP does not exclude the condition. HLA-B27 supports the diagnosis but is not diagnostic.
• MRI of the sacroiliac joints is now recommended as the first imaging test, in place of X-ray (2025 EULAR imaging update, presented at ACR Convergence 2026).
• Refer to rheumatology early. Diagnosis is often delayed by years.

“Some patients come to us after years of treatment aimed at a mechanical cause for their back or buttock pain. Before planning any intervention, I ask whether the pattern could be inflammatory, because that changes who should be leading the patient’s care.”

Dr Murli Krishna, Consultant in Pain Medicine, Pain Spa

Group	SpA features to look for
Back and buttock	Inflammatory back pain; alternating buttock pain
Peripheral	Asymmetric arthritis; heel pain (enthesitis); dactylitis
Outside the joints	Anterior uveitis (iritis); psoriasis; inflammatory bowel disease
Family history	axSpA, reactive arthritis, psoriasis, inflammatory bowel disease or anterior uveitis
Response and bloods	Good response to NSAIDs; raised CRP or ESR

Feature	Inflammatory pattern	Mechanical pattern
Onset	Insidious, usually before 40	Any age, often activity-related
Exercise	Improves	Worsens with activity, flexion, sitting or a prolonged position
Rest	No improvement	Relieved
Night	Night pain, often waking in the second half of the night	Less typical
Morning stiffness	Prolonged, more than 30 minutes	Brief or absent
Site	Alternating buttock or deep gluteal pain	Unilateral or localised; may radiate to buttock or posterior thigh, typically not past the knee
NSAIDs	Marked improvement	Variable

Area	What to assess
Spinal mobility	Spinal tenderness; lumbar range of motion (Schober test); chest expansion, judged against age and sex
Posture	Occiput-to-wall distance and thoracic kyphosis (later findings)
Sacroiliac joints	Pain reproduced by provocation tests such as FABERE or Gaenslen
Peripheral	Peripheral joints; entheses (Achilles tendon, plantar fascia); digits for dactylitis
Skin and nails	Psoriasis
Eyes	A painful red eye with photophobia needs urgent ophthalmology referral for slit-lamp examination to rule out acute anterior uveitis
Gut	Diarrhoea, blood in the stool, weight loss

Test	What it tells you	Caution
HLA-B27	Positive in 70–90% of patients with axSpA. Supports the diagnosis in the right clinical setting.	A positive result does not mean the patient has axSpA. Most carriers never develop it, so the result only carries weight when the clinical picture fits. Not a screening test.
CRP and ESR	Supportive when raised.	Raised in only about 40% of patients, so a normal result does not exclude disease. Also raised by obesity, infection and other inflammatory arthritis.
Baseline bloods	Full blood count, renal and metabolic profile, liver function. Help with the differential and prepare for NSAID or biologic therapy.	Rheumatoid factor and anti-CCP only if peripheral joint findings suggest rheumatoid arthritis.

“A normal CRP is the result that most often gives false reassurance. If the history fits, normal bloods should not close the question.”

Dr Murli Krishna, Consultant in Pain Medicine, Pain Spa

What has changed

The 2025 update of the EULAR imaging recommendations, presented at ACR Convergence 2026, makes MRI of the sacroiliac joints the first imaging test for diagnosing axSpA, in place of X-ray. MRI of the spine is added when there are symptoms in that region.

Data from the SPACE early-disease cohort, also presented at ACR Convergence 2026, show that sacroiliac joint MRI lesions are highly specific for early axSpA (95% or higher), with sensitivity of 36–51%. A positive MRI is therefore strong evidence, but a negative MRI does not fully exclude the diagnosis.

Test	Role	Key points
MRI sacroiliac joints	First imaging test (EULAR 2025 update)	STIR and T1 sequences. Detects active inflammation before structural change appears on X-ray. Gadolinium adds little.
MRI spine	Added when there are spinal symptoms	Can also help with the differential diagnosis.
X-ray pelvis or sacroiliac joints	Previously first-line. Now an alternative when MRI is unavailable or contraindicated	Definite sacroiliitis (bilateral grade 2 or higher, or unilateral grade 3–4) meets the modified New York criteria. Changes can take years to appear and readers often disagree.
X-ray spine	Not part of the diagnostic work-up	Not recommended for diagnosis.

“A normal X-ray was never good evidence against early disease, because the changes take years to show. Going straight to MRI of the sacroiliac joints gives a patient with a convincing story a much better chance of an early answer.”

Dr Murli Krishna, Consultant in Pain Medicine, Pain Spa

Reading this with the 2025 guidance: where MRI of the sacroiliac joints is the first test, a positive MRI answers the imaging question at the outset. The feature count and HLA-B27 then support the clinical diagnosis.

Adapted from the ASAS modification of the Berlin diagnostic algorithm, as shown in Mease P, Deodhar A. Differentiating nonradiographic axial spondyloarthritis from its mimics: a narrative review. BMC Musculoskelet Disord 2022. SpA features are listed in the table under “Who to Evaluate”.

Entry criterion: back pain for 3 months or more, onset before age 45
Imaging arm Sacroiliitis on imaging plus at least 1 SpA feature	Clinical arm HLA-B27 positive plus at least 2 other SpA features

Condition	Distinguishing points
Mechanical or degenerative spine disease	The most common mimic
Diffuse idiopathic skeletal hyperostosis (DISH)	Older patients; flowing anterior ossification; sacroiliac joints spared
Osteitis condensans ilii	Sclerosis on the iliac side; often in women after childbirth
Fibromyalgia or chronic widespread pain	Widespread pain without objective inflammation
Infection	Unilateral sacroiliitis with fever or systemic signs (septic sacroiliitis, brucellosis, tuberculosis)
Malignancy	History of cancer; weight loss

Red flags for serious spinal pathology

These call for prompt evaluation or imaging, not a routine inflammatory back pain work-up:

• Progressive motor or sensory deficit
• Bowel or bladder dysfunction
• Fever or unexplained weight loss
• History of cancer
• Recent or current infection
• Significant trauma
• Prolonged corticosteroid use

In a patient with established disease and new spinal pain, stay alert to an unstable vertebral fracture and immobilise properly before imaging.

“Our role is to recognise the pattern, arrange the right tests and involve a rheumatologist early. Pain management still has a place afterwards, but it works best alongside treatment of the underlying disease.”

Dr Murli Krishna, Consultant in Pain Medicine, Pain Spa

Most back pain is mechanical, meaning it comes from the joints, discs and muscles and is not caused by inflammation. A small number of people have an inflammatory condition called ankylosing spondylitis, which affects the joints at the base of the spine and the spine itself. It usually starts in the teens, twenties or thirties.

Signs that are worth mentioning to your doctor

• Back pain that has lasted more than three months and started before you were 45
• Pain and stiffness that are worse in the morning or after resting, and ease when you move
• Pain that wakes you in the second half of the night
• Buttock pain that swaps from side to side
• Pain that improves a lot with anti-inflammatory tablets
• A history of psoriasis, bowel inflammation, a painful red eye, heel pain or a swollen finger or toe
• A close relative with a similar condition

What tests might I have?

• Blood tests. These look for inflammation and for a gene called HLA-B27. They are helpful but cannot give the answer alone. Blood tests are often normal even when the condition is present, and many people carry the gene without ever becoming unwell.
• An MRI scan. This is now the preferred first scan because it can show inflammation early, before anything is visible on an X-ray.
• A specialist opinion. The diagnosis is made by a rheumatologist, who puts your symptoms, examination, blood tests and scan together.

Why does early diagnosis matter?

Many people wait years for the right diagnosis. Finding it sooner means the right treatment can start sooner.

Seek urgent medical help if you have a painful red eye with sensitivity to light, new weakness or numbness, loss of bladder or bowel control, or fever with back pain.