Frequent detection of anti-nuclear antibodies and low-titer rheumatoid factor in female patients with undifferentiated peripheral spondyloarthritis in Japan: a prospective observational study.

Objectives: Studies on the clinical features of undifferentiated peripheral spondyloarthritis (upSpA) are limited. This study explores sex-based manifestations of upSpA and compares upSpA with rheumatoid arthritis (RA) under conditions of low human leukocyte antigen B27 positivity in Japan.

Methods: In this multicenter prospective observational study, physical and laboratory findings from 29 upSpA cases (19 females, 10 males) were collected to investigate sex differences. Fourteen patients with RA were also enrolled as a control group to evaluate clinical parameters distinguishing upSpA and RA.

Results: Tenderness at the humeral epicondyles and the Achilles tendon were more common in female patients with upSpA than in males. Serum C-reactive protein (CRP) levels were lower in female patients with upSpA than in male patients. Serum rheumatoid factor (RF) levels were lower in upSpA cases than in RA, with a cut-off value of 84 IU/mL differentiating RA from upSpA. An anti-nuclear antibodies (ANA) test at a dilution of ≥1:160 was detected in about half of the female upSpA cases but not in male cases.

Conclusions: Among upSpA and RA cases, female patients with upSpA frequently present with multiple enthesitis, low serum CRP levels, low-titer RF positivity, and a high incidence of ANA positivity.