Which color tube is used for a PT (prothrombin time) test?

• A. Red
• B. Green
• C. Light blue
• D. Lavender

Answer: (C)

Prothrombin time relies on a plasma sample collected with an anticoagulant that preserves clotting factors. The light blue top tube contains sodium citrate, which binds calcium and prevents clotting during transport and handling. This keeps the sample suitable for accurately measuring how long the clotting process takes. It’s crucial to fill the tube to the correct volume to maintain the proper citrate-to-blood ratio; otherwise the PT result can be distorted. After centrifugation, the plasma is used in the PT assay, with calcium reintroduced by reagents to initiate clotting in the test. That’s why the light blue tube is the one used for a PT test. Red tubes have no additive and are used for serum chemistry, green tubes contain heparin for other plasma tests, and lavender tubes contain EDTA for hematology; these additives would interfere with coagulation testing.