According to the ESSG criteria, for a patient to be classified as having SpA, he or she has to satisfy one of two entry criteria: Inflammatory spinal pain OR synovitis that is either asymmetric or predominantly in the lower limbs.

• Inflammatory back pain: Back pain is common among the general population. However, "inflammatory" back pain is much less common. Back pain is considered inflammatory if four of the following five criteria are found:

1. Onset of back discomfort before the age of 40 years

2. Insidious onset

3. Persistence for at least three months

4. Associated with morning stiffness

5. Improvement with exercise

• Asymmetrical synovitis: Asymmetrical synovitis, predominantly of the lower limbs is manifested by soft tissue swelling, warmth over a joint, joint effusion, and reductions in both active and passive range of motion. As with inflammatory spinal pain, the symptoms are worse after a period of rest.

Additional criteria: If a patient has one or both of the entry criteria listed above, he or she should then be evaluated for the presence of one or more of the following features:

1. Positive family history

2. Psoriasis

3. Inflammatory bowel disease

4. Urethritis, cervicitis, or acute diarrhea within one month before arthritis

5. Buttock pain alternating between buttocks

6. Enthesopathy

7. Plain film radiographic evidence of sacroiliitis

Importantly, blood tests, including an assessment for the presence of HLA-B27, are not part of the ESSG criteria; in addition, only the sacroiliac joints need to be evaluated radiographically.