Which of the following tubes is used for serum collection without anticoagulants?

The red plastic tube is specifically designed for serum collection without the presence of anticoagulants. This tube allows for the blood to clot naturally after collection, which is essential for the separation of serum from the cellular components of the blood once the sample is centrifuged.

In contrast, the other tubes contain anticoagulants or additives that prevent clotting: the green tube contains heparin, the light blue tube contains sodium citrate, and the purple tube contains EDTA. These additives are used for different types of tests that require whole blood or plasma rather than serum, which is the liquid portion obtained after blood has fully clotted. Therefore, for tests that necessitate a serum sample, such as certain chemistry panels, the red plastic tube is the appropriate choice as it ensures that no anticoagulant interferes with the results.