A catheter unit (200) for use in an intravenous catheter instrument (1000) is provided. The catheter unit (200) comprises a catheter hub (201) and a catheter (202) extending distally from the catheter hub (201). The catheter (202) has a lumen (203) being in flow communication with an interior cavity (204) of the catheter hub (201). The interior cavity (204) is provided with a cooperation groove (205) at a proximal zone of the interior cavity (204). A catheter instrument (1000) comprising such a catheter unit (200) is also provided.
