Which tube contains no additives for serum separation?

The red tube is specifically designed to contain no additives, which makes it ideal for collecting serum samples. When blood is drawn into a red-top tube, it clots, and after centrifugation, the serum can easily be separated from the clot. This is critical for various laboratory tests that require serum analysis, as any additives could potentially interfere with accurate results.

In contrast, the other tubes mentioned contain additives. The gold tube, for example, typically contains a gel separator along with serum separation properties, while the light green tube usually has lithium heparin as an additive, which is used for plasma collection. The purple tube contains EDTA, an anticoagulant that prevents blood from clotting by binding calcium ions. These additives play essential roles in certain types of tests but are not appropriate for scenarios where pure serum is required. Thus, the correct choice is the red tube, as it allows for the collection of serum without any contaminants affecting the sample.