What is a PT test?

• A. A lipid panel using red tube
• B. A coagulation study using light blue tube with sodium citrate
• C. A CBC using purple tube
• D. A metabolic panel using green tube

Answer: (B)

A PT test is a blood coagulation study that measures how long it takes for plasma to form a clot, focusing on the extrinsic pathway of coagulation. The blood is drawn into a light blue top tube containing sodium citrate, which binds calcium to prevent clotting during transport. In the lab, citrate is removed and calcium plus tissue factor are added to trigger clot formation, and the time is measured. This test reflects the activity of factors II, V, VII, and X and is commonly used to monitor warfarin therapy and assess vitamin K deficiency, liver disease, or factor deficiencies. Results are reported in seconds and often as an INR. Other common panels use different tube colors: a lipid panel uses a red top, a CBC uses a purple top, and a metabolic panel uses a green top.