The present invention provides a method of analyzing plasma from a sample of whole blood. The method includes a) obtaining a sample of whole blood from a subject in a container, the container having a first end and a second end, the first end of the container for receiving a sample of whole blood; b) centrifuging the container under conditions to separate the whole blood into a cell layer, a gel layer and a plasma layer, wherein the centrifuging produces in relative juxtaposition the first end of the container, the cell layer, the gel layer, the plasma layer and the second end of the container; and c) inserting into a spectrophotometric device the second end of the container which includes the plasma layer in closest relative juxtaposition, to analyze the plasma from the whole blood.