The invention provides methods and apparatus for detecting an analyte in blood. The apparatus is particularly suited for bringing a sensor into direct contact with blood in vivo. The apparatus comprises a sensor that detects the presence of an analyte and an assembly means. The assembly means has a sensor end, wherein the sensor end of the assembly means is affixed to the sensor, and the assembly means is adapted for coupling with a venous flow device. By coupling with a venous flow device, the assembly means brings the sensor into direct contact with blood flowing through the venous flow device. Examples of venous flow devices that bring the sensor into direct contact with the blood of a subject include, but are not limited to, intravenous catheters and external blood loops, such as are used in extra corporeal membrane oxygenation or hemodialysis.