Anti-U1-RNP antibodies and mixed connective tissue disease: a real-world diagnostic phenotype

Luana Stanciu⁽¹⁾, Claudiu Popescu^(1,2,*), Mihaela Agache^(1,2), Catalin Codreanu^(1,2)

1)Carol Davila University of Medicine and Pharmacy, Bucharest, Romania
2)Ion Stoia Clinical Center for Rheumatic Diseases, Bucharest, Romania

Background. The study aims to observe real-life diagnostic outcome of patients testing positive for anti-U1-RNP antibodies and to identify clinical features useful for distinguishing mixed connective tissue disease (MCTD).
Methods. Data of all patients who were admitted to a tertiary university rheumatology hospital within the last 5 years (Oct 15th 2019 – Nov 18th 2024) and who were tested quantitatively for anti-U1-RNP antibodies were retrospectively collected. All diagnoses and clinical manifestations were according to each attending senior rheumatologist.
Results. Only 36 patients (5.4%) had positive anti-U1-RNP antibodies. These patients were in most cases women (88.9%), with a median age of 44 years. The median titers of positive anti-U1-RNP antibodies were 200 U/mL. Only 66.7% were diagnosed with MCTD. Compared to non-MCTD patients, MCTD patients were significantly older, with higher median anti-U1-RNP antibody titers, and higher prevalence of Raynaud’s phenomena, puffy hands/fingers, and rheumatoid factor positivity, lower prevalence of thrombocytopenia, positive anti-dsDNA antibodies, positive anti-Sm antibodies and hypocomplementemia. Only 38.9% of positive anti-U1-RNP antibody patients fulfilled the Alarcón-Segovia and Villareal criteria (ASVC). Only 54.2% of patients diagnosed with MCTD fulfilled the ASVC.
Conclusion. The predominant phenotype of patients with positive anti-U1-RNP antibodies and a clinical diagnosis of MTCT includes female sex, fifth decade of life, arthritis, hand edema, Raynaud’s phenomena, esophageal involvement and inflammatory syndrome. Clinical judgment is more inclusive than the ASVC for MCTD and it seems to be oriented towards MCTD by higher titers of anti-U1-RNP antibodies and toward SLE by positive anti-dsDNA and anti-Sm antibodies.

Keywords: mixed connective tissue disease, anti-U1-RNP antibodies, phenotype

Corresponding author(s): Claudiu Popescu, Ion Stoia Clinical Center for Rheumatic Diseases, Bucharest, Romania; email: claudiu.popescu@reumatologiedrstoia.ro