Final answer:
A red top tube is used to collect a serum sample; during an hour in the tube, blood clots, separating serum for analysis such as glucose levels and blood typing.
Step-by-step explanation:
The tube that is normally used to collect a serum sample is the red top tube. This type of tube allows blood to clot, and the serum is obtained from the liquid portion of the blood after the clot has formed and been removed. During the hour that the blood sample is in the glass tube, the blood cells clot, and the serum separates from the clotted cells. The serum is harvested for analysis which can include measurements of glucose levels, diagnose diseases, and classify blood types.