Final answer:
The clear yellow colored portion in a centrifuged blood specimen without anticoagulant is called serum, which is used for many diagnostic tests and lacks coagulation factors.
Step-by-step explanation:
The clear yellow colored portion in a centrifuged blood specimen collected in a vacuum tube without anticoagulant is called serum. When blood is centrifuged, the cells and platelets move to the bottom and above this layer is the buffy coat, which contains leukocytes and platelets. Above the buffy coat is the plasma. If the blood sample has not been treated with an anticoagulant, the clotting factors in the plasma will act, and the liquid portion that remains after the blood has clotted and been centrifuged is referred to as serum, which still contains various substances such as antibodies, but lacks coagulation factors compared to plasma. Serum is commonly used for various diagnostic tests because it is free of blood cells and clotting factors.