Abstract: A catheter for intravascular use has a blood inlet and a blood outlet, and includes a membrane arranged in the catheter in such a way that at least one part of the blood flowing into the catheter via the blood inlet during operation comes into contact with the membrane. The membrane allows an exchange of at least one substance between a carrier medium and the blood. The carrier medium is a carrier fluid in which the substance to be exchanged can be dissolved, and the catheter includes a delivery device that is designed to at least partially compensate for a pressure difference between the blood inlet and the blood outlet during operation. A method for removing at least one substance from venous blood for diagnostic purposes uses a device of this type.