Systems and method are disclosed for determining a concentration of an analyte in a fluid (e.g., blood). The system can draw blood from a patient and deliver the blood to a sample cell. A particular component of the fluid (e.g., plasma) may be separated and/or positioned such that the concentration of the analyte is measured in the particular component of the fluid (e.g., plasma). The sample cell can include a sample container that has two window pieces. The system can have a fluid passage having a tip configured to mate with a multi-lumen catheter without leaking. The multi-lumen catheter can have proximal and distal ports. A fluid pressure system can be configured to periodically draw fluid from vasculature through a proximal intravascular opening and the proximal port while maintaining a low pressure and/or flow rate to thereby reduce risk of reversing the fluid flow in a vessel and drawing infusates upstream into another intravascular opening.