Which tube is not supposed to contain any additive?

The red top tube is designed specifically to be a plain tube without any additives. This is crucial for certain laboratory tests that require serum to be collected without the influence of any anticoagulants or other chemicals that could interfere with the results. The red top tube allows for the blood to clot naturally, resulting in serum that can be then separated by centrifugation for various analyses.

In contrast, the other tubes listed contain additives that serve specific purposes: the green top tube typically contains heparin for anticoagulation, the light blue top tube contains sodium citrate which is also an anticoagulant, and the lavender top tube usually contains EDTA, which helps prevent clotting and is often used for hematology tests. Knowing which tubes contain additives and which do not is essential for ensuring accurate test results.