Method and system for monitoring of blood constituents in vivo

A method for determining the concentration of a constituent of blood employing a sensor in or near a catheter in a blood vessel for producing a signal related to the concentration of the constituent of blood is disclosed. The improvement comprises obtaining the signal while at least one of the following is present in the blood vessel: (1) an added vasodilator component and/or vasodilation promotor in an amount effective to reduce the vasoconstriction caused by the presence of the catheter in the blood vessel and (2) an added platelet/white cell inhibitor component and/or a platelet/white cell deactivation promotor in an amount effective to reduce the platelet and/or white cell activation caused by the presence of the catheter in the blood vessel.