In some examples, a device includes a catheter insert elongated body defining a body lumen, the catheter insert elongated body being configured to be at least partially inserted to a catheter lumen defined by a catheter without covering a first fluid opening of the catheter and to form a fluidically tight coupling with the catheter, and one or more sensors positioned on the elongated body. At least one of the one or more sensors are configured to sense a substance of interest. The catheter insert elongated body includes a material that is a substantially non-permeable to the substance of interest.