Which tube color top should be used for routine clinical chemistry (serum) testing?

Tube Color for Routine Clinical Chemistry (Serum) Testing

For routine clinical chemistry testing, use gold-topped or tiger-topped serum separator tubes (SST), which contain clot activators and gel separators that provide optimal serum quality for electrolyte and chemistry panels. 1

Standard Tube Selection

• Serum separator tubes (SST) are the gold standard for routine chemistry testing, typically identified by gold/yellow tops or tiger-striped (red/gray) tops 1
• These tubes contain clot activators (usually silica particles) that accelerate the clotting process and a gel separator that forms a physical barrier between serum and cellular components after centrifugation 1
• The gel barrier prevents contamination of serum with cellular components, which is critical for accurate electrolyte and chemistry measurements 1

Proper Collection and Processing Protocol

• Allow 30 minutes clotting time for SST tubes at room temperature before centrifugation (compared to 60 minutes for plain red-top tubes without gel) 1
• Gently invert the tube 5-8 times immediately after collection according to manufacturer recommendations to ensure proper mixing of blood with the clot activator 1
• Complete processing within 2 hours of collection to minimize pre-centrifugation delays that can affect test accuracy 1

Order of Draw Considerations

When collecting multiple tube types, follow this sequence to prevent cross-contamination: 2

1. Blood culture tubes (first)
2. Sodium citrate tubes (light blue)
3. Serum tubes (gold/tiger-top SST) - third position
4. Heparin tubes (green)
5. EDTA tubes (lavender/purple)
6. Sodium fluoride/potassium oxalate tubes (gray)

Critical Advantages of SST Over Plain Tubes

• SST tubes remain stable after centrifugation and do not require immediate serum transfer, unlike plain red-top tubes where serum must be separated to prevent ongoing cellular metabolism 3
• The gel separator has been validated for 81 different chemical, toxicological, and immunological tests with clinically acceptable accuracy 4
• Modern SST formulations (SST II Plus) demonstrate excellent stability for up to 72 hours of storage, making them superior for delayed testing 5

Common Pitfalls to Avoid

• Insufficient clotting leads to fibrin formation that can clog analyzers, while prolonged clotting causes cell lysis and falsely elevated potassium values 1
• Underfilling tubes disrupts the proper blood-to-additive ratio and compromises gel barrier formation 2
• Patients on anticoagulant therapy may require longer clotting times than the standard 30 minutes 1

Special Note on Serum vs. Plasma

• Serum (from SST tubes) does not contain anticoagulants that might interfere with certain electrolyte measurements, particularly potassium, making it preferred over plasma for routine chemistry panels 1
• While plasma tubes (green-top heparin or lavender-top EDTA) process faster, serum provides more accurate results for most chemistry tests and remains stable after separation 6, 7