Objective To determine the sensitivity and specificity of enthesis ultrasound for the diagnostic classification of early spondyloarthritis.

Methods A cross-sectional, blinded and controlled study. Standardised bilateral ultrasound of six entheses (Madrid sonography enthesitis index (MASEI)) was performed. Accepted diagnostic classification criteria were used as the gold standard. Validity was analysed by receiver operating characteristic (ROC) curves. Values of p<0.05 were considered significant.

Results 113 early spondyloarthritis patients were included (58 women/55 men), 57 non-inflammatory control individuals (29 women/28 men) and 24 inflammatory control individuals (11 women/13 men). The evolution time of spondyloarthritis was 10.9±7.1 months. At least some grade of sacroiliitis on x-ray was present in 59 patients, but only five fulfilled the radiographic sacroiliitis New York criteria. Human leucocyte antigen B27 (HLA-B27) was positive in 42% of patients. No statistical differences were found for the enthesis score among diagnostic spondyloarthritis subtypes form of presentation (axial, peripheral or mixed) or HLA-B27 positivity. The MASEI score achieved statistical significance for gender. The ultrasound score was 23.36±11.40 (mean±SD) in spondyloarthritis patients and 12.26±6.85 and 16.04±9.94 in the non-inflammatory and inflammatory control groups (p<0.001), respectively. The ROC area under the curve was 0.82, and a cut-off point of ≥20 points achieved a likelihood ratio of 5.30 and a specificity of 89.47%.

Conclusions Entheses are affected early in spondyloarthritis, and the incidence of involvement is higher in men and independent of the spondyloarthritis diagnostic subtype, HLA-B27 status or presentation pattern. The enthesis ultrasound score seems to have diagnostic accuracy and may be useful for improving the diagnostic accuracy of early spondyloarthritis.