SST Tube vs Empty Tube for Next-Day Testing
For blood samples requiring next-day testing, use an SST (Serum Separator Tube) rather than an empty tube, as the gel barrier prevents cellular contamination and maintains analyte stability for up to 24 hours when properly refrigerated. 1
Why SST Tubes Are Superior for Delayed Testing
The SST tube's gel separator creates a physical barrier between serum and cellular components after centrifugation, which is critical for preventing analyte degradation during overnight storage 2, 3. This barrier prevents:
- Hemolysis from prolonged cell contact, which falsely elevates potassium, LDH, and other intracellular components 4
- Continued metabolic activity of blood cells that can alter glucose, electrolytes, and other analytes 5
- Cellular contamination during subsequent handling and testing 3
Proper Collection and Processing Protocol
Initial Processing (Day of Collection)
- Allow 30 minutes clotting time at room temperature with the tube sitting upright before centrifugation 2, 1, 3
- Complete centrifugation within 2 hours of collection at 1500-2000 × g for 10 minutes 1
- After centrifugation, the gel forms a stable barrier that maintains serum integrity 4, 5
Overnight Storage
- Refrigerate at 4°C immediately after centrifugation—this is essential for maintaining biomarker stability 2, 1
- Serum can remain stable in the SST tube for up to 24 hours when refrigerated 1
- Keep the tube sealed and upright to maintain the gel barrier integrity 3
Why Empty Tubes Fail for Delayed Testing
Empty tubes (plain red-top tubes without gel) require:
- 60 minutes clotting time (twice as long as SST) 2, 3
- Manual serum separation into a secondary storage tube, introducing contamination risk 3
- No barrier protection against continued cellular contact, leading to progressive analyte changes including neuron-specific enolase elevation and complement degradation 4, 6
Critical Pitfalls to Avoid
- Temperature control is non-negotiable: Room temperature storage accelerates degradation exponentially compared to refrigeration, particularly for labile analytes like Aβ peptides 2, 1
- Never exceed 24 hours refrigerated storage before testing; beyond this timepoint, freeze at -80°C if testing must be further delayed 1
- Avoid -20°C storage for clinical purposes—only -80°C provides adequate long-term preservation 1
- Monitor for lipemia: Cloudy serum indicates lipemia that interferes with spectrophotometric measurements and requires specimen rejection 7
Analyte-Specific Stability
Research demonstrates that SST tubes maintain stability for 42 commonly ordered analytes over 72 hours, including tumor markers, electrolytes, and special proteins 4, 5, 6. The only exceptions requiring caution are: