APPLICATION OF THE NEW ACR/EULAR 2019 CLASSIFICATION CRITERIA OF SYSTEMIC LUPUS ERYTHEMATOSUS TO A INCEPTION, MONOCENTRIC COHORT OF UNDIFFERENTIATED CONNECTIVE TISSUE DISEASES POPULATION AT ONSET OF THE DISEASE

Background: undifferentiated connective tissue diseases (UCTDs) are a group of systemic autoimmune diseases that share clinical and serological manifestations with definite connective tissue diseases (CTDs), but do not satisfy existing classification criteria . Within this group, it is possible to identify very heterogeneous conditions: transitory and self-limiting forms, stable conditions over time and forms that will evolve towards definite CTDs, in particular Systemic Lupus Erythematosus (SLE). The availability of new classification criteria for CTDs could be useful in identifying major CTDs from their onset and help in the differentiation from stable UCTDs, which will maintain their undifferentiated profile over time. Objectives: the aim of this study was to apply the new ACR/EULAR 2019 classification criteria of SLE to patients included, at the onset of UCTD, to evaluate how many patients could be re-classified as SLE. Methods: this is a retrospective observational study that enrolls patients who received the diagnosis of UCTD at the Rheumatology Unit of Pisa, according to the classification criteria proposed by Mosca et al , and were then regularly followed in the same clinic (inception cohort). For each patient, demographics, comorbidities, treatment, clinical and serological data were collected, at baseline and during follow-up. The new ACR/EULAR 2019 classification criteria of SLE were applied to the inception cohort at the onset of the disease. The characteristics of patients re-classified as SLE according to the new classification criteria and those of patients who “remained” classified as UCTD were compared. Results: we enrolled 202 patients with a diagnosis of UCTD, mainly female (F:M 193:9) and of Caucasian ethnicity (97.5%), mean age at the diagnosis 38,5 ± 13,2 years and a median follow-up of 5 years (IQR 2-10). During the follow-up, 10 patients (4.9%) in our cohort developed clinical and serological characteristics that led to a diagnosis of a definite CTD, in particular: 7 SLE, 2 Mixed Connective Tissue Disease (MCTD) and 1 Sjogren’s syndrome. Applying the ACR/EULAR 2019 classification criteria of SLE to patients enrolled at the onset of the disease, 38/202 (18.8%) would have been classified as SLE (with a median score of 12 (IQR 11-15)) on the basis of the presenting clinical and serological manifestations. Interestingly, 6/7 patients who received a diagnosis of SLE during the follow-up were among these 38 patients. Among the items of the new classification criteria of SLE, the most frequently satisfied by patients “re-classified” were arthritis (63.2%), hematological (44.7%) and skin (23.7%) manifestations; as for the “immunological” items, hypocomplementemia (71%), SLE-specific (52.6%) and antiphospholipid (35.1%) autoantibodies. Comparing the subgroup of patients “re-classified” as SLE with that of patients who “remained” UCTD, we found that the first group presented more frequently hematological manifestations, hypocomplementemia and anti-dsDNA, anti-Sm, anti-RNP, anti-beta2GPI positivity at the onset of the disease (p<0.01). Moreover, during the follow-up, the subgroup of patients “re-classified” as SLE developed more frequently malar (p<0.05) and discoid rash and arthritis (p<0.01) compared to patients who “remained” UCTD. Conclusion: The development of increasingly sensible and specific classification criteria for definite CTDs may guide in the identification of stable UCTDs since their early stages and consequently in better definition of these conditions that can be considered as a distinct clinical entity. The early identification of stable UCTD is of great importance not only for clinical management (follow-up schedules as well as therapeutic protocols) but also for scientific implications. REFERENCES: [1]Mosca M. et al., Clin Exp Rheumatol. Sep-Oct 1999;17(5):615-20; 2. Fanouriakis A. et al., Ann Rheum Dis. 2020 Jun;79(6):713-723. Disclosure of Interests: None declared Citation: Ann Rheum Dis, volume 80, supplement 1, year 2021, page 647Session: SLE, Sjögren’s and APS - clinical aspects (other than treatment) (POSTERS only)