Vacutainer Tube for Protein C and Protein S Testing
Use a light blue top tube containing 3.2% (0.105-0.109 mol/L) trisodium citrate at a 9:1 blood-to-anticoagulant ratio for Protein C and Protein S testing.
Tube Specifications
The appropriate collection tube is:
- Light blue top vacutainer with sodium citrate anticoagulant
- Specific concentration: 3.2% (0.105-0.109 mol/L) trisodium citrate
- Fill ratio: One-tenth volume of citrate to nine-tenths volume of blood 1
Critical Collection Requirements
Fill Volume
- Fill the tube to the optimal volume marked on the tube
- Underfilling or overfilling will alter the blood-to-anticoagulant ratio and invalidate results
- The citrate concentration must be precisely maintained for accurate coagulation testing
Sample Processing
After collection, the sample requires specific handling 1:
- Double-centrifuge the specimen to achieve platelet-poor plasma
- Target: residual platelet count <10.0 × 10⁹/L
- This rigorous platelet removal is essential for thrombophilia assays including Protein C and Protein S
Timing Considerations
- Test plasma within 4 hours of collection, OR
- Freeze at -20°C if testing within 2 weeks
- Freeze at -70°C or below if storing >2 weeks
- Thaw frozen samples rapidly at 37°C in a waterbath before testing
Common Pitfalls to Avoid
Tube fill errors: The most critical error is improper fill volume. Even the minimum fill volume in citrate tubes can slightly affect coagulation results 2, though this primarily impacts PT and APTT rather than Protein C/S directly. Always aim for optimal fill.
Wrong tube type: Do not use serum separator tubes (SST, RST) or plasma separator tubes (PST) with gel barriers for coagulation studies—these are designed for chemistry testing, not coagulation assays 3, 4, 5.
Inadequate centrifugation: Single centrifugation or insufficient speed leaves residual platelets that can interfere with Protein C and Protein S assays. The double-centrifugation requirement distinguishes thrombophilia testing from routine coagulation studies 1.