What is the purpose of drawing a red tube before a light blue topped tube?

The reasoning behind the correct answer is rooted in the specific characteristics of the additives contained in different types of blood collection tubes. The light blue-topped tube is typically used for coagulation tests and contains sodium citrate, which acts as an anticoagulant. This means it can interfere with other tests if the blood is contaminated by substances from other tubes.

By drawing a red-topped tube first, which contains no additive, you help minimize the risk of contamination from potential clotting factors or other substances that might be present in the collection system beforehand. This practice ensures that the blood sample collected in the light blue tube remains uncontaminated by any substances that could falsely influence the coagulation test results. Therefore, this careful order of draw facilitates accurate testing and reliable results for coagulation assessments.