POS1131 EVOLUTIONARY TRAJECTORY OF UNDIFFERENTIATED CONNECTIVE TISSUE DISEASE: IMPACT OF THE 2019 EULAR/ACR CLASSIFICATION CRITERIA FOR SYSTEMIC LUPUS ERYTHEMATOSUS
Authors: C. Ciancarella, F. Ceccarelli, L. Picciariello, F. Natalucci, A. I. Celia, C. Garufi, S. Mancuso, S. Truglia, A. Gattamelata, F. R. Spinelli, C. Alessandri, F. Conti
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> Undifferentiated connective tissue disease (UCTD) is a condition characterized by serological evidence of autoimmunity and occurrence of clinical symptoms suggestive of systemic autoimmune diseases, yet not fulfilling specific classification/diagnostic criteria. From an evolutionary point of view, three possible situations can be observed: (1) the so-called UCTD could represent an early stage of a specific autoimmune disease, such as Systemic Lupus Erythematosus (SLE); (2) UCTD could remain stable without progressing to another disease; (3) UCTD could resolve over time, with the disappearance of clinical and serological symptoms. The early identification of patients at risk to develop a defined disease could help to introduce an appropriate treatment and to prevent chronic damage. Finally, the update of different classification criteria could modify the UCTD epidemiology due to the possibility to capture more patients affected by specific autoimmune diseases. <h3>Objectives:</h3> In the present longitudinal, observational, retrospective study we aimed to analyze the evolution of UCTD course, focusing on the impact of 2019 EULAR/ACR classification criteria for SLE. <h3>Methods:</h3> Since 2008 we have consecutively collected data about patients diagnosed with UCTD. All subjects were evaluated every six months or in accordance with the patient's clinical course, to record the development of clinical and laboratory features suggestive for specific autoimmune diseases. The present analysis was restricted to UCTD patients with a follow-up in our clinic of at least 6 months. Indeed, we retrospectively applied the 2019 EULAR/ACR SLE classification criteria at the first and last visit in our out-patient clinic. <h3>Results:</h3> We included 201 UCTD patients [F/M 191/10, median age at first visit 50 years (IQR 19.5), median disease duration at first visit 6 years (IQR 7.5)]. At the first visit, 27 patients (13.4%) already met the 2019 SLE EULAR/ACR classification criteria. In these patients, the most frequent clinical manifestation was joint involvement (70.4%), followed by thrombocytopenia (29.6%) and positivity for antiphospholipid antibodies (29.6%) <b>(Figure 1</b>). We found a significant association between SLE classification and thrombocytopenia (p=0.0001), serositis (p= 0.02), joint involvement (p=0.03), low levels of C3 and C4 (p=0.03, p=0.0001 respectively), positivity for anti-dsDNA/antiSm (p=&lt;0.0001). Logistic regression analysis confirmed the association with thrombocytopenia (OR 25.4, p&lt;0.0001), anti-dsDNA/anti-Sm positivity (OR 18.6, p=0.001), low C4 levels (OR 12.7, p=0.02), joint involvement (OR 4.9, p=0.01). At the first visit, 16 patients (59.2%) were taking a treatment (12 patients hydroxychloroquine, 4 glucocorticoids). At the last visit, 5 patients were lost to follow-up; the proportion of treated patients increased to 95.4%, with the introduction of immunosuppressive agents in 4 patients. According to the SLICC Damage Index (SDI), at the first visit 2 patients (7.4%) showed damage in at least one organ/system; this proportion increased to 18.2% at the last assessment. We longitudinally followed the remaining 174 UCTD patients (<b>Figure 2</b>). During a mean observation period of 45.9 ± 35.6 months, 33 patients (18.9%) were lost to follow-up, while 141 patients were followed. At the last visit, additional 11 patients (7.8%) could be classified as having SLE, three Sjögren Syndrome, 1 psoriatic arthritis, and 1 rheumatoid arthritis (according to specific classification criteria). Indeed, 125 patients (88.6%) remained UCTD and 3 patients (2.1%) experienced a resolution of clinical and serological manifestations. <h3>Conclusion:</h3> The results of our analysis confirmed that a relevant proportion of UCTD patients could be re-classified as having SLE according to the most recent classification criteria. Thrombocytopenia, antiDNA/antiSm positivity and low C4 levels represent the most associated factors. However, it should be considered the possible evolution to other autoimmune conditions and serological/clinical remission in a small proportion of patients. <h3>REFERENCES: NIL.</h3> <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>