Which of the following is not a standard vacuum tube size?

In the context of blood collection using vacuum tubes, standard sizes typically include options such as 5 ml, 10 ml, and 15 ml. Each of these sizes serves specific laboratory needs, with 5 ml tubes being common for certain tests and the larger 10 ml and 15 ml tubes allowing for greater volume collection when required for specific assays.

While 20 ml tubes may exist, they are not considered a standard size widely used in practice compared to the other options listed. Numerous laboratory protocols specify using 5 ml, 10 ml, and 15 ml tubes for ease of handling and consistency in testing. Typically, the standardization of sizes helps ensure compatibility with collecting equipment and analytical techniques used in the laboratory setting. Thus, among the provided options, the 20 ml size stands out as less commonly utilized in standard laboratory settings, confirming it as the correct choice in this question.