To clarify the influence of human leucocyte antigen B27 (HLA-B27) status on the phenotype of early axial spondyloarthritis (SpA).

708 patients with inflammatory back pain (IBP) defined by Calin or Berlin criteria were recruited; 654 fulfilled at least one of the SpA criteria (modified New York, European Spondyloarthropathy Study Group, Amor or Assessment of SpondyloArthritis international Society classification criteria for axial SpA) and were included in the analyses. Clinical, demographic and imaging parameters were compared between HLA-B27 positive and negative groups. Significant parameters in univariate differences between HLA-B27 positive and negative groups were retested in multivariate models explaining various outcomes.

Patients had a short duration of axial symptoms (mean 1.5 years) and HLA-B27 was present in 61.5%. In multivariate analysis, HLA-B27 positivity was associated with a younger age at onset of IBP (regression coefficient (B)=(-2.60), p<0.001), less delay in diagnosis (B=(-1.02), p=0.01), lower frequency of psoriasis (OR 0.59, p=0.01) and higher frequency of MRI inflammation of the sacroiliac joints (SIJ) (OR 2.13, p<0.001), MRI inflammation of the spine (OR 1.59, p=0.04) and radiographic sacroiliitis (OR 1.56, p=0.03). MRI inflammation of the SIJ was shown to be an intermediate variable between HLA-B27 positivity and radiographic sacroiliitis.

In early axial SpA, HLA-B27 is associated with earlier onset of IBP, less delay in diagnosis, axial inflammation (spine and SIJ), radiographic damage of the SIJ, decreased disease activity and lower frequency of psoriasis. It is not associated with physical function and MRI structural lesions of the SIJ.