In our study we assessed, for the first time, the prognostic role of US in combination with ST biopsy in foreseeing the future differentiation into defined arthritis in seronegative UPIA patients showing that, at baseline, PDUS and GSUS scores are directly associated with histological synovial inflammation in terms of CD68+, CD3+ cells and CD31+ vessels. Here, PECAM1 is linked to Arthritis.