The current ASAS classification of AxSpA relies either on sacroiliitis on imaging plus one SpA feature (imaging arm) or HLA-B27 antigen plus two SpA features (clinical arm), in a patient with chronic back pain and age at onset of less than 45 years. IBP which is a major symptom of SpA depends more on patient's perception which is not usually accurate. As well, disease activity is measured by ASDAS, BASDAI, and BASFAI which depend more on subjective measures. Assessment of reliability of IBP criteria, ASDAS, BASDAI, and BASFAI in diagnosis and evaluation of activity of AxSpA is essential for better health care.

Axial spondyloarthritis (AxSpA) is a chronic inflammatory disease that affects the axial skeleton mainly leading to significant pain and disability. Early diagnosis of AxSpA is important for better management of symptoms and to limit spinal damage. The last classification of AxSpA is ASAS classification criteria which relies either on sacroiliitis on imaging plus one SpA feature (imaging arm) or HLA-B27 antigen plus two SpA features (clinical arm), in a patient with chronic back pain and age at onset of less than 45 years. Inflammatory back pain (IBP) is the key clinical symptom of AxSpA and is often present at disease onset. There are different sets of IBP criteria which include the Calin criteria , Berlin criteria, and the Assessment of SpondyloArthritis international Society (ASAS) IBP experts criteria. These sets when assessed by different rheumatologists had different sensitivity and specificity.

As IBP has certain characters, taking the clinical history has been considered as a screening test for patients with chronic back pain to identify patients with AxSpA. But, the value of this screening test has been questioned. Disease activity in AxSpA has been measured by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). It has been used extensively since 1994 but with some limitations as it does not include the physician's assessment of the disease and does not assess the impact of specific clinical factors.

The Bath Ankylosing Spondylitis Functional Index (BASFI) has been also developed as an approach to defining and monitoring functional ability in patients with AxSpA. ASAS developed a disease activity score for patient with AxSpA which is called Ankylosing Spondylitis Disease Activity Score (ASDAS) that combines patient-reported assessments with acute phase reactants (C-reactive protein [CRP] or erythrocyte sedimentation rate [ESR]). However, it is based largely on subjective measures, and lacks objective biomarker of disease activity.

The evaluation of disease activity in AxSpA is complex and multifactorial. Also, patients and physicians have different perspectives of the disease and none of the current single-item or combined indexes adequately unifies both perspectives.

Inclusion Criteria:

• Group 1
• Patients diagnosed as AxSpA.
• Age > 18 years and < 45 years at onset of the disease. Group 2
• Patients diagnosed as chronic mechanical back pain.
• Age > 18 years and < 45 years at onset of the disease.

Exclusion Criteria:

• Patients having acute or subacute back pain.
• Patients having back pain referred from another site.
• Patients with any other autoimmune disease: rheumatoid arthritis, systemic lupus erythematosus, Behçet's disease… etc.

Group 1 Patients diagnosed as AxSpA according to the ASAS classification criteria for AxSpA. Group 2 Patients diagnosed as chronic (more than 3 months) mechanical back pain, the diagnosis has been made prior to the study by the treating specialist, these patients are herein as a control group.