An SST blood test isn’t a single test. SST stands for Serum Separator Tube, which is a specific type of blood collection tube used to run a wide range of common lab tests. If your lab order mentions an SST, it means your blood will be drawn into a tube designed to cleanly separate the liquid portion of your blood (serum) from the cells, so the lab can analyze your blood chemistry, check organ function, screen for infections, and more.
What the Tube Actually Does
An SST tube has two key components inside it. The first is a clot activator, typically tiny silica particles coating the inner wall. When your blood enters the tube, these particles trigger your body’s natural clotting process, causing the blood to solidify. The second component is a special gel that sits at the bottom of the tube.
After your blood clots (usually about 30 minutes), the tube is placed in a centrifuge and spun at high speed for around 15 minutes. During spinning, the gel shifts upward and wedges itself between the clot and the clear, straw-colored serum above it. This creates a physical barrier that keeps the two layers permanently separated. The serum sitting on top of the gel is what the lab actually tests.
This matters because serum gives a cleaner sample than whole blood for many types of analysis. Unlike plasma (which is separated using chemicals that prevent clotting), serum contains no anticoagulant additives that could interfere with results. It’s considered the gold standard for many chemistry and metabolic tests.
How to Recognize an SST Tube
SST tubes are commonly called “gold top” tubes because of their gold-colored cap. You may also hear them called “tiger top” tubes, referring to an older version with a mottled red-and-gray cap. Both serve the same purpose. If you see a gold-capped tube during your blood draw, that’s the SST.
Tests Commonly Run From an SST
The SST tube covers a surprisingly broad range of diagnostic tests. The most common ones fall into a few categories.
Metabolic and Chemistry Panels
A basic metabolic panel (BMP) checks your blood sugar, kidney function, and electrolyte balance. A comprehensive metabolic panel (CMP) adds liver function markers and protein levels. Both are frequently drawn into SST tubes. Lipid panels, which measure your cholesterol and triglyceride levels, also use serum from an SST.
Organ Function Tests
Liver function tests, kidney panels, and markers like uric acid and bilirubin are routinely measured from SST serum. The gel barrier in SST tubes actually helps keep certain analytes stable longer. Glucose, blood urea nitrogen, uric acid, and bilirubin all show better stability in gel tubes compared to plain tubes, especially when samples are refrigerated.
Infectious Disease and Antibody Tests
SST tubes are the standard collection method for serology testing, which checks whether your immune system has produced antibodies against specific infections. A single 5 mL SST tube can provide enough serum for up to six different antibody tests. The list of infections commonly screened this way includes hepatitis A, B, and C, HIV, syphilis, herpes (HSV types I and II), Epstein-Barr virus, cytomegalovirus (CMV), measles, mumps, rubella, chickenpox, and toxoplasmosis.
Thyroid and Hormone Tests
Thyroid panels and many hormone level checks also use serum collected in SST tubes. The clean separation the gel provides is important for the immunoassay techniques labs use to measure hormone concentrations.
Do You Need to Fast?
It depends on which specific test your provider ordered, not on the tube itself. Blood glucose tests, lipid panels, and basic metabolic panels typically require 8 to 12 hours of fasting beforehand. Fasting means no food or drinks besides plain water. Coffee, juice, flavored water, and sodas can all affect results. You should also avoid chewing gum, smoking, and exercise during the fasting window.
Antibody tests, liver function panels, and many other SST-drawn tests usually don’t require fasting. Your lab order or provider’s office will specify if fasting is needed.
Sample Stability and Timing
One practical advantage of the SST tube is that the gel barrier helps keep your sample accurate for longer. Research shows that common analytes like albumin, total protein, creatinine, cholesterol, triglycerides, and several enzyme markers remain stable for at least three days after centrifugation, whether stored at room temperature or refrigerated. HDL cholesterol stays stable for up to 36 hours. Refrigeration at 4°C extends stability further for glucose, bilirubin, and uric acid.
This stability window means that even if there’s a delay between your blood draw and when the lab processes your sample, results from an SST tube remain reliable. For routine bloodwork, the tube is spun within one hour of collection, and the gel barrier does its job from that point forward.
Why Your Lab Uses SST Instead of Other Tubes
Labs prefer SST tubes for serum-based tests because they streamline the process. The tube only needs to be centrifuged once, the gel eliminates the need to manually transfer serum to a separate container, and the sealed barrier reduces the risk of contamination. The tube can go directly into automated analyzers with a single label, which cuts down on errors.
Some specialized tests, like certain drug level monitoring or advanced metabolic profiling, may require a different tube type because the gel or silica particles can occasionally interfere with very sensitive detection methods. Your provider or lab will choose the right tube for each test automatically, so this isn’t something you need to worry about or request.