The soluble CD18 (sCD18) plasma concentration, HLA genotype, and spondyloarthritis (SpA) disease activity. (A) The sCD18 plasma level in HLA-B27-positive versus HLA-B27-negative SpA patients at the time of inclusion. The median values were 605.7 mU/mL (interquartile range 461.6-704.1 mU/mL) for HLA-B27-positive SpA patients and 670.0 mU/mL (interquartile range 591.8-760.1 mU/mL) for HLA-B27-negative SpA patients. Bars indicate median and interquartile range. (B) Correlation between the sCD18 plasma levels in HLA-B27-positive SpA patients and self-assessment and clinical scores. (C) Correlation between the sCD18 plasma levels in HLA-B27-positive SpA patients and clinical test results. Solid lines represent the best fit in linear regression. *P <0.05; **P <0.01; ***P <0.001; ****P <0.0001.