SST Tube Centrifugation After Visual Clot Formation
Even if blood appears fully clotted visually after 15 minutes, you should wait the full 30 minutes at room temperature before centrifugation to ensure complete clot formation and prevent fibrin interference with laboratory testing. 1, 2
Recommended Clotting Time Before Centrifugation
- SST tubes require a minimum of 30 minutes of upright standing at room temperature after blood collection to allow proper clot formation, regardless of visual appearance 1, 2
- The 30-minute clotting period is critical because visual assessment of clot formation is unreliable—incomplete clotting can lead to fibrin strands in serum that interfere with automated analyzers and produce inaccurate results 1, 2
- If you cannot centrifuge immediately after the 30-minute clotting period, refrigerate the tubes at 4°C for no longer than 2 hours to prevent biomarker degradation 1, 2
Proper Centrifugation Protocol
Once the 30-minute clotting time is complete:
- Centrifuge at 1500-2000 × g for 10 minutes at 4°C to achieve adequate serum separation while minimizing hemolysis 1, 3, 2
- Use the lowest deceleration setting on your centrifuge to prevent resuspension of the cellular layer 3
- Avoid collecting the last 0.5 cm of serum above the clot to minimize cellular contamination 3
Critical Timing Considerations
- The entire process from blood collection to centrifugation should be completed within 2 hours (including the 30-minute clotting time) to prevent analyte degradation 1, 2
- After centrifugation and serum separation, test samples within 2 hours or store at 4°C for up to 4 hours maximum before testing 2
- For longer storage needs, freeze serum at -80°C immediately after separation 2
Common Pitfalls to Avoid
- Never centrifuge before 30 minutes have elapsed, even if the clot looks solid—premature centrifugation causes incomplete clot retraction and fibrin contamination 1, 2
- Do not exceed 2 hours of total pre-centrifugation time at room temperature, as this accelerates biomarker degradation exponentially compared to refrigerated storage 2
- If you observe cloudy or turbid serum after centrifugation, this indicates lipemia or incomplete separation and the specimen should be rejected and recollected with proper patient preparation 4