CRP and ankylosing spondylitis: Vitamin D may play a protective role in AS based on a positive correlation between the serum vitamin D level and disease severity.54 There is a significant negative correlation between the vitamin D level and disease activity indicated by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level.55 However, there are conflicts regarding the relationship between the vitamin D level and AS disease activity.55,56 The cause of low vitamin D levels in patients with AS is also largely unclear.