Blood Collection Tube for Routine Electrolyte Testing
For routine serum electrolyte measurement, use serum separator tubes (SST) with gold/yellow or tiger-top color coding, which contain clot activators and gel separators. 1
Recommended Tube Type and Rationale
Serum separator tubes (SST) are the standard collection method for electrolyte testing as recommended by the American Society for Clinical Pathology. 1 These tubes contain:
- Clot activators that accelerate the clotting process 1
- Gel separators that form a physical barrier between serum and blood cells after centrifugation, preventing cellular contamination that can affect electrolyte measurements 1
The gel barrier is particularly important because it prevents potassium leakage from cells, which is a common source of falsely elevated potassium results. 1
Proper Collection Technique
Tube Filling
- Fill tubes completely until the vacuum is exhausted to maintain the proper blood-to-anticoagulant ratio 2
- Underfilled tubes cause spurious results and are the most common preventable error in electrolyte testing 2
Clotting Time
- Allow 30 minutes clotting time for SST tubes before centrifugation 1
- Plain red-top tubes (without gel separator) require 60 minutes clotting time 1
- Insufficient clotting leads to fibrin formation that can clog analyzers, while prolonged clotting causes cell lysis and contamination 1
Mixing
- Gently invert the tube according to manufacturer recommendations immediately after collection to ensure proper mixing with the clot activator 1
Processing Timeline
Complete processing within 2 hours of collection to minimize pre-analytical errors. 1 After centrifugation, the gel separator creates a stable barrier that improves analyte stability compared to non-separator tubes. 3
Alternative: Plasma Tubes (Less Preferred for Electrolytes)
While lithium heparin plasma tubes can technically be used for electrolytes, they have significant disadvantages:
- Serum is preferred over plasma because it contains no anticoagulants that might interfere with electrolyte measurements, particularly potassium 1
- Plasma separation tubes show reduced stability for potassium (10-13 hours) compared to serum gel tubes (60+ hours) 4
- Heparin tubes require complete filling; underfilling causes spurious variations in potassium and other analytes 5
Critical Pitfalls to Avoid
- Never use EDTA (purple-top) or citrate (blue-top) tubes for electrolytes, as these anticoagulants chelate calcium and alter sodium measurements 2
- Avoid hemolyzed specimens, as cell lysis releases intracellular potassium and falsely elevates results 2
- Do not store samples open at room temperature beyond 3 hours, as evaporation and cellular leakage cause clinically significant changes in sodium, chloride, and potassium 6
- Patients on anticoagulant therapy may require longer clotting times before centrifugation 1
Order of Draw
When collecting multiple tubes, draw the serum tube (SST) third in the sequence: blood culture first, sodium citrate second, then serum tubes, followed by heparin and EDTA tubes. 2 This prevents cross-contamination of additives between tubes.