CPT (Certified Phlebotomy Technician) Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

The correct choice for coagulation studies is the light blue top tube. This tube contains sodium citrate, an anticoagulant that is specifically designed to prevent blood from clotting during the testing process. The use of sodium citrate is crucial because it maintains the blood's ability to undergo coagulation tests like PT (Prothrombin Time) and APTT (Activated Partial Thromboplastin Time) in a reliable manner.

When performing coagulation studies, it is essential to properly fill the light blue top tube to the indicated volume, as the ratio of blood to anticoagulant is critical for accurate test results. If the tube is underfilled, it can alter the anticoagulant effect, leading to inaccurate results.

Other tubes, such as the red top, green top, and purple top, contain different additives geared toward other types of tests—like serum biochemistry or hematology—but are not suitable for coagulation testing due to their respective compositions.