CRP and ankylosing spondylitis: 25(OH)D 25-hydroxyvitamin D, BMI body mass index, p25 − p75 25th to 75th percentile, ESR erythrocyte sedimentation rate, CRP C-reactive protein, RF rheumatoid factor, ACPA anti-cyclic citrullinated peptide antibodies, DAS28-ESR, disease activity score using 28 joints-erythrocyte sedimentation rate, HAQ (0–3): health assessment questionnaire, HLA-B27 histocompatibility antigen HLA-B27, BASDAI (0–10) Bath ankylosing spondylitis disease activity index, BASFI (0–10) Bath ankylosing spondylitis functional index, DMARD disease-modifying anti-rheumatic drugs