In the order of draw recommended by CLSI, which tube is collected third?

In the order of draw recommended by CLSI (Clinical and Laboratory Standards Institute), the correct sequence is designed to prevent cross-contamination of the additive from one tube to another. In this context, the third tube to be collected is the heparin tube, which is represented by the green tube.

The order of draw typically starts with blood cultures, followed by coagulation tubes (often blue), and then serum tubes (red). Collecting serum tubes after coagulation tubes prevents any potential contamination of the serum sample that could affect the results, as serum tubes do not contain any anticoagulant.

Thus, the correct answer is that the heparin tube is collected third, indicating its position in the sequence which prioritizes the integrity of test results by minimizing interference from additives in the tubes.