Final answer:
A medical assistant should allow a blood sample to clot without anticoagulants before centrifuging. This process separates the serum from the clotted materials in the blood, useful for diagnostic testing.
Step-by-step explanation:
A medical assistant should allow a serum specimen to clot without an anticoagulant additive to stand for a set period of time prior to centrifuging. During the time the sample is in the tube, typically about an hour, the blood clots and the cellular components become separated from the plasma. Once clotting has occurred, centrifuging the specimen will result in the separation of the serum from the clotted components. The serum, which is the plasma with the clotting factors removed, can then be harvested for analysis.
Adding an anticoagulant such as potassium oxalate, K2C2O4, to a blood sample will prevent clotting by removing calcium ions necessary for coagulation. However, for tests requiring serum, no anticoagulants are used so that clotting can take place. After centrifuging, the clear serum can be used for various diagnostic tests to measure substances like glucose or calcium concentration in the blood.