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BACKGROUND: Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by significant clinical heterogeneity with early diagnosis being a major challenge, complicated by the absence of formal diagnostic criteria. Instead, classification criteria have been developed to enable the homogenous inclusion of patients in clinical trials, with the most commonly used those of the American College of Rheumatology (ACR 1997) and the Systemic Lupus International Collaborating Clinics Classification Criteria (SLICC 2012). These criteria are widely used in clinical practice as diagnostic tools, although they fail to diagnose up to 20% of patients with SLE or may delay diagnosis. These restrictions have led to the recent (2018) introduction of new classification criteria jointly by the European League Against Rheumatism (EULAR) and ACR. AIMS OF THE STUDY: We will compare the sensitivity and specificity of the earlier and new classification criteria after a systematic analysis (retrospective study) of a group of SLE patients. In addition, we will examine which set of criteria permits the earliest classification of the disease in a prospective cohort of patients with undifferentiated connective tissue disease (UCTD). The prognostic impact (permanent organ damage) of the classification of SLE patients with the three sets of criteria will also be examined. METHODS: Data from the existing Cretan lupus registry will be used to retrospectively include consecutively registered patients aged ≥15 years diagnosed with SLE during 01/2005-12/2016 by an expert physician and followed-up for at least 6 months. All sets of criteria (ACR 1997, SLICC 2012, EULAR/ACR 2018) will be tested at the time of physician-based diagnosis and also at last follow-up. A prospective study arm will include cases with a diagnosis of UCTD and will be followed-up in the outpatient clinic for 3-5 years. ANTICIPATED BENEFITS: This is the first study to include the application of the new criteria (EULAR/ACR 2018) to a group of SLE patients. Determining their diagnostic value in comparison to existing criteria or diagnosis by a specialist will provide important information both for the value of their application at the level of clinical studies and for their use in clinical practice as diagnostic criteria.