A catheter instrument, comprising a needle tip shielding device, a needle, a needle carrying unit, and a catheter unit, said catheter unit comprising a catheter hub and a catheter, and said needle tip shielding device and said catheter hub being separable from one another; wherein - said needle comprising a needle tip; - said catheter hub comprises: a catheter hub distal end and a catheter hub proximal end, said catheter hub distal end having said catheter extending therefrom, and a catheter hub opening, said catheter hub opening defining a catheter hub annular space; - said needle carrying unit further comprising: a needle carrying unit distal end and a needle carrying unit proximal end, said needle carrying unit distal end having said needle extending therefrom; - such that when said needle is in the ready position, in which said needle projects into said catheter, said needle is partly disposed and slidable within said needle tip shielding device; - such that when said needle is in a fully retracted position, in which said needle tip is completely withdrawn from said catheter, said needle tip shielding device is distally shielding said needle tip and - a stop member, preventing said needle tip shielding device from distal movement relative to said needle in the fully retracted position; characterized in that - said needle tip shielding device is kept in contact with said catheter unit in said ready position via at least one interface between said needle tip shielding device and said catheter unit; - said needle tip shielding device being of a first polymeric material and said catheter hub being of a second polymeric material, wherein an interface surface of said needle tip shielding device and an interface surface of said catheter hub forms said at least one interface between said needle tip shielding device and said catheter unit, and wherein said first polymeric material is a thermoplastic polymer comprising covalently bonded O or S atoms and said second polymeric material is polypropylene, polyethylene, or propylene/ethylene co-polymer. - said needle comprising a needle tip; - said catheter hub comprises: a catheter hub distal end and a catheter hub proximal end, said catheter hub distal end having said catheter extending therefrom, and a catheter hub opening, said catheter hub opening defining a catheter hub annular space; - said needle carrying unit further comprising: a needle carrying unit distal end and a needle carrying unit proximal end, said needle carrying unit distal end having said needle extending therefrom; - such that when said needle is in the ready position, in which said needle projects into said catheter, said needle is partly disposed and slidable within said needle tip shielding device; - such that when said needle is in a fully retracted position, in which said needle tip is completely withdrawn from said catheter, said needle tip shielding device is distally shielding said needle tip and - a stop member, preventing said needle tip shielding device from distal movement relative to said needle in the fully retracted position; - said needle tip shielding device is kept in contact with said catheter unit in said ready position via at least one interface between said needle tip shielding device and said catheter unit; - said needle tip shielding device being of a first polymeric material and said catheter hub being of a second polymeric material,