Disclosed is a catheter (10) and associated method for taking a plurality of samples from within a length of a blood vessel (2). The catheter includes an elongate central body (12) arranged to be inserted into and positioned along a central region of a blood vessel. A plurality of collection areas (14) are defined along the elongate central body for collecting samples at the central region of the blood vessel. A plurality of mixers (18) are provided radially outwardly of the elongate central body and arranged to create a flow of blood from a boundary layer at a wall of the blood vessel to the elongate central body. This enables the collection areas to collect samples from the boundary layer.
