19th January 2024

Diagnosis 

The diagnosis of AS can be challenging, as symptoms may be diverse, and patients may present with either musculoskeletal or extra-articular complaints or both. According to NICE guidelines, AS should be suspected in patients below the age of 45 years who present with lower inflammatory back pain and stiffness lasting more than 3 months. It should also be suspected in the presence of buttock, thoracic or cervical pain, asymmetric and peripheral arthritis, enthesitis, anterior uveitis, or in those with psoriasis, inflammatory bowel disease or genitourinary infections. Symptoms typically wake patients up during the night and respond to NSAIDs. There may be a family history of AS or other spondyloarthritides. As the diagnosis can be difficult, referral to a rheumatologist is recommended.

Classifications

Modified New York criteria for ankylosing spondylitis:

The ‘Modified New York Criteria’ or the ‘Assessment of Spondyloarthritis International Society (ASAS) Classification Criteria’ can be used by rheumatologists as an aid for the diagnosis of AS.

A diagnosis of AS can be made in the presence of criterion 4 plus any one of the other criteria:

1. Low back pain of at least 3 months’ duration that is improved by exercise and not relieved by rest.
2. Limited lumbar spinal motion in sagittal and frontal planes.
3. Chest expansion decreased relative to normal values for sex and age.
4. Bilateral sacroiliitis grade 2 to 4, or unilateral sacroiliitis grade 3 or 4.

ASAS classification criteria of axial spondyloarthritis

The Assessment of Spondyloarthritis (ASAS) International Society has developed the following criteria for the classification of AS for patients under the age of 45 years, with back pain lasting more than 3 months:

Either

• Sacroiliitis on MRI or x-ray imaging
• Plus at least 1 spondyloarthritis feature from:
  • Inflammatory back pain
  • Arthritis
  • Heel enthesitis
  • Uveitis
  • Dactylitis
  • Psoriasis
  • Crohn’s/colitis
  • Good response to NSAIDs
  • Family history of spondyloarthritis
  • HLA-B27
  • Elevated C-reactive protein

or

• HLA-B27-positive
• Plus at least 2 spondyloarthritis features

Investigations

The diagnosis of AS cannot be made reliably based on a single test, and often, a number of investigations are carried out, in addition to suggestive clinical features, in order to confirm the diagnosis.

Blood tests:

• Useful for ruling out other diagnoses mimicking AS (see differential diagnoses section below).
• FBC: could reveal normocytic normochromic anaemia of chronic disease
• ESR and CRP should be checked, and may be raised in AS, although a negative result does not rule out the disease
• HLA B-27 testing can be performed in patients presenting with highly suggestive features of AS prior to referral to rheumatology
• Rheumatoid factor and antinuclear antibodies are not necessary to make a diagnosis

Image Source: Wikimedia Commons (2020). Bamboo spine ankylosing spondylitis, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Bamboo_spine_ankylosing_spondylitis.jpg (Accessed: 30 June 2021).

If X-rays are negative for sacroiliac joint involvement, but there remains a high index of suspicion for AS, an MRI should be requested. MRI may be useful in early disease to identify sacroiliitis, which would present as bone marrow oedema.

Spirometry may be useful for diagnosing restrictive lung disease in patients with apical fibrosis, kyphosis and involvement of the costovertebral joints.