Systems and method are disclosed for determining a concentration of an analyte (e.g., glucose) in a fluid (e.g., blood). The system can draw blood from a patient and deliver the blood to a sample cell. A centrifuge motor can spin the sample cell to separate the fluid into a plurality of components (plasma, red blood cells, etc.). A particular component of the fluid (e.g., plasma) may be positioned at a sample portion of the sample cell after centrifuging 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 cuvette that has two window pieces sandwiched between two clamshell pieces, and where the sample portion of the sample cell is defined by a gap between the window pieces.