Which of the following tubes will yield a serum sample?

A red top tube is designed to yield a serum sample due to its inherent properties. This tube typically contains no additives, allowing blood to clot naturally when it is drawn. After the blood has clotted, the tube is then centrifuged, separating the serum from the clotted cells. Since serum is the liquid portion of blood that remains after coagulation, the red top tube is particularly useful for tests that require serum, such as biochemical tests, certain serological tests, and hormone levels.

In contrast, the blue top tube contains sodium citrate as an anticoagulant, which prevents the blood from clotting and is primarily used for coagulation studies, resulting in plasma rather than serum. The green top tube typically contains heparin, another anticoagulant, which also results in plasma samples. Lastly, the gray top tube is used for glucose testing and may contain additives like sodium fluoride, which inhibits glycolysis, again yielding plasma rather than serum. Therefore, the red top tube is the only option listed that yields serum upon proper processing.