Which tube color is typically used for coagulation studies such as prothrombin time (PT)?

• A. Red
• B. Light blue
• C. Purple
• D. Gray

Answer: (B)

Coagulation tests like PT rely on obtaining plasma with an anticoagulant that preserves clotting factors without altering them. The tube used for this is the light blue top, which contains sodium citrate. Sodium citrate binds calcium to keep blood from clotting, and the tube must be filled to the correct blood-to-anticoagulant ratio (about 9:1). After collection, the sample is centrifuged to obtain platelet-poor plasma, and the PT is measured by adding thromboplastin and calcium. If the tube isn’t filled properly or the wrong type is used, the citrate-to-blood ratio changes and can skew clotting times, leading to inaccurate results. Red tops are for serum without anticoagulant, purple tops use EDTA for complete blood counts, and gray tops contain fluoride-oxalate for glucose testing, not coagulation studies. So, the color used for coagulation tests like PT is the light blue tube.