Which of the following tubes should not contain any additives?

The red top tube is the correct choice because it is designed specifically for the collection of serum and does not contain any additives. This makes it ideal for tests that require serum samples, as the absence of additives means that the blood can clot naturally, allowing for the serum to be separated from the clotted blood via centrifugation.

In contrast, the gold top tube contains a gel separator and clot activators, which assist in the clotting process and separation of serum. The purple top tube, also known as the lavender tube, is used for hematology tests and contains EDTA, an anticoagulant that prevents clotting. The light blue top tube is used for coagulation studies and contains sodium citrate, which also acts as an anticoagulant, preventing blood from clotting during testing. Thus, the presence of additives in these tubes serves specific purposes for various types of tests, while the red top tube remains additive-free, allowing for a straightforward clotting process to yield serum.