investigations

Last edited 12/2020 and last reviewed 10/2023

The fundamental investigations in ankylosing spondylitis are:

• FBC - leucocytosis
• ESR - elevated
• CRP - raised
• hypergammaglobulinaemia
• negative rheumatoid factor

Specialist laboratories may also be able to determine whether a patient has histocompatability complex, HLA-B27, which is strongly associated with ankylosing spondylitis.

Imaging for suspected axial spondyloarthritis
Initial investigation using X-ray (1)

• diagnose radiographic axial spondyloarthritis (ankylosing spondylitis) if the plain film X-ray shows sacroiliitis meeting the modified New York criteria (bilateral grade 2–4 or unilateral grade 3–4 sacroiliitis)
• if the plain film X-ray does not show sacroiliitis meeting modified New York criteria (bilateral grade 2–4 or unilateral grade 3–4 sacroiliitis), or an X-ray is not appropriate because the person's skeleton is not fully mature, request unenhanced MRI using an inflammatory back pain protocol

Subsequent investigation using MRI (1)

• radiologists receiving a request for an inflammatory back pain MRI should perform short T1 inversion recovery (STIR) and T1 weighted sequences of the whole spine (sagittal view), and sacroiliac joints (coronal oblique view)
• use the ASAS/Outcome Measures in Rheumatology (OMERACT) MRI criteria to interpret the MRI as follows:
• If the MRI meets the ASAS/OMERACT MRI criteria:
• diagnose non-radiographic axial spondyloarthritis
• If the MRI does not meet the ASAS/OMERACT MRI criteria:
• do not exclude the possibility of axial spondyloarthritis
• consider specialist musculoskeletal radiology review if there is disparity between the clinical suspicion and imaging findings, particularly in people with an immature skeleton
• offer an HLA-B27 test if it has not already been done. If positive, base the diagnosis of non-radiographic axial spondyloarthritis on clinical features, for example, using the clinical 'arm' of the ASAS axial classification criteria

If a diagnosis of axial spondyloarthritis cannot be confirmed and clinical suspicion remains high, consider a follow-up MRI.

Notes:

• it has been stated that the progression of radiological features and measures of acute phase response (e.g. ESR, C-reactive protein concentrations) poorly reflect symptoms in ankylosing spondylitis (2)
  • thus it has been suggested that clinical assessment be based mainly on validated questionnaires usually completed by patients such as the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), which measures pain, tenderness, stiffness and fatigue, and the Bath Ankylosing Spondylitis Functional Index (BASFI), which measures activities of daily living (3,4)

Reference:

• NICE (2017). Spondyloarthritis in over 16s: diagnosis and management
• Drug and Therapeutics Bulletin 2005; 43 (3):19-22.
• Garrett S et al.A new approach to defining disease status in ankylosing spondylitis: The Bath Ankylosing Spondylitis Disease Activity Index. J Rheumatol 1994; 21: 2286-91.
• Calin A et al. A new approach to defining functional ability in ankylosing spondylitis: the development of the Bath Ankylosing Spondylitis Functional Index. J Rheumatol 1994; 21: 2281-5.