ASCP (American Society for Clinical Pathology) Phlebotomy Technician Certification Practice Exam

The correct procedure involves using a discard tube before collecting a sodium citrate tube for PT and PTT tests because this technique helps to ensure accurate test results. During the butterfly collection method, it is common to experience sample contamination or to have air in the tubing, especially with smaller volume devices like butterfly needles. By using a discard tube first, any potential contaminating materials or air that may be taken up during the initial venipuncture can be eliminated, ensuring that the sample collected in the subsequent sodium citrate tube is as clean and representative of the patient's blood as possible. This is particularly critical for coagulation studies such as PT (Prothrombin Time) and PTT (Partial Thromboplastin Time), where precise ratios of blood to anticoagulant are essential for accurate results.

The other choices do not adhere to this critical step of using a discard tube, which is necessary to mitigate contamination or incorrect volume in the sodium citrate sample collection.