An SST blood test isn’t a specific test. SST stands for Serum Separator Tube, which is a type of blood collection tube used to run many of the most common lab tests your doctor orders. If you see “SST” on your lab paperwork, it refers to the gold-topped tube your blood was drawn into, not the test itself. That tube is designed to cleanly separate the liquid portion of your blood (serum) from the cells, giving the lab a pure sample to analyze.
What’s Inside an SST Tube
An SST tube has two key components. The first is a clot activator, typically a silica-based coating on the inner wall of the tube. When your blood enters the tube, this coating speeds up the natural clotting process. The second is a layer of thixotropic gel sitting at the bottom, a special substance that behaves like a solid when still but flows when force is applied.
After the blood clots, the tube goes into a centrifuge, a machine that spins it at high speed. During spinning, each component of the blood separates based on its density. The heavier red blood cells and clot sink to the bottom. The lighter serum rises to the top. The gel, which has a density right between the two, migrates upward and lodges between them, forming a stable physical barrier. This keeps the serum completely isolated from the cells below, even during transport to the lab.
Why Serum and Not Whole Blood
Most chemistry tests your doctor orders require serum rather than whole blood. Serum is the clear, yellowish liquid left after blood has clotted and the cells and clotting proteins have been removed. It’s different from plasma, which is obtained by adding an anticoagulant to prevent clotting. Both contain the substances labs need to measure, but serum tends to have higher concentrations of small molecules, making it more sensitive for detecting biomarkers. It also has lower protein content, which can improve accuracy for certain analyses.
Which Tests Use an SST Tube
The SST tube is one of the most versatile tubes in the lab. It’s used for a wide range of routine blood work, including:
- Comprehensive metabolic panel (CMP): kidney function, liver enzymes, blood sugar, electrolytes
- Basic metabolic panel (BMP): a smaller subset of the CMP focusing on glucose, calcium, and kidney markers
- Lipid panel: cholesterol and triglycerides
- Thyroid function tests: TSH, T3, T4
- Iron studies and vitamin levels
- Hormone levels: testosterone, estrogen, cortisol
- Certain antibody and immune markers
If you’ve had routine blood work at an annual physical, there’s a good chance at least one of your tubes was an SST.
How to Identify an SST Tube
SST tubes are recognizable by their gold cap. Some older or alternative versions have a red-and-gray speckled top (sometimes called a “tiger top”). A plain red-top tube also collects serum but lacks the gel barrier, meaning the lab has to manually transfer the serum to a separate container. The gold SST eliminates that step, reducing handling time and the risk of contamination.
How the Sample Is Handled
After your blood is drawn into an SST tube, the phlebotomist gently inverts it about six times to mix the blood with the clot activator. Then it sits undisturbed for a minimum of 30 minutes to allow a complete clot to form. Rushing this step can leave tiny clot fragments in the serum, which interfere with test results.
Once clotting is complete, the tube is centrifuged. Standard protocols spin the tube for 10 minutes at moderate speed, though some labs use a faster spin for a shorter duration (as little as 5 minutes) with equivalent results. After spinning, the gel barrier is in place and the serum on top is ready for testing. Labs aim to separate the serum from cells within two hours of collection.
Once separated, the serum stays stable at room temperature for up to 8 hours. If testing will take longer, the sample is refrigerated, where it remains stable for about 48 hours.
Where SST Falls in the Draw Order
If you’ve ever had multiple tubes of blood drawn in a single sitting, there’s a specific sequence labs follow to prevent cross-contamination between tube additives. SST tubes are drawn third, after blood culture bottles and blue-topped coagulation tubes. They come before tubes containing anticoagulants like heparin or EDTA. This order matters because the silica clot activator in SST tubes can interfere with coagulation testing if it carries over into a blue-top tube.
Limitations of SST Tubes
SST tubes work well for most routine chemistry, but they aren’t appropriate for every test. The gel barrier can absorb certain drugs and compounds from the serum over time, which makes SST tubes unreliable for some therapeutic drug monitoring. The silica clot activator has also been shown to affect measurements of certain immune proteins involved in the complement pathway. For tests sensitive to these interferences, labs will specify a different tube type, such as a plain red-top or a heparin tube.
Complete blood counts (CBCs) and coagulation studies like PT/INR are never run from an SST tube. CBCs require whole blood collected in a lavender EDTA tube, and coagulation tests need the blue citrate tube. If your lab order includes both a CBC and a metabolic panel, you’ll have at least two different tubes drawn.