Which type of specimen is most often collected in a red plastic tube?

The correct choice is serum, which is typically collected in a red plastic tube. Red plastic tubes often contain a clot activator that facilitates the clotting process, allowing for the separation of serum from the blood cells after centrifugation. When blood is drawn into these tubes, the blood coagulates, and following centrifugation, the liquid portion (serum) can be carefully separated from the solid components (clotted cells).

In contrast, whole blood is not collected in a red plastic tube because it is intended to retain all components of blood, including cells and plasma. Plasma is collected in different types of tubes, often those that contain anticoagulants to prevent clotting, so it is not appropriate for red plastic tubes. Urine collections are entirely separate from blood collection and typically require different containers, such as sterile urine cups, making them an irrelevant choice for this question.