The existence of systemic autoimmune diseases not fulfilling classification criteria for defined connective tissue diseases (CTDs) is a common clinical experience. These conditions have been variably defined as incomplete lupus erythematosus, early undifferentiated connective tissue diseases and undifferentiated connective tissue diseases (UCTDs). However, the definition of UCTD includes a wide spectrum of diseases ranging from 'organ-dominant' conditions (e.g., idhiopatic non-specific interstitial pneumonia) to simplified conditions (stable UCTD), to early CTDs or mild forms of CTDs. In the present article, the literature data on undifferentiated diseases and their clinical spectrum as well as the importance of the definition of new classificative criteria are discussed.
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