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General product information only, not medical advice. Collection, handling and testing follow the tube manufacturer’s instructions for use (IFU) and the testing laboratory’s own requirements.
A serum separator tube (SST) is a blood collection tube that combines a clot activator with a gel barrier. After the blood clots and the tube is centrifuged, the gel settles between the clotted cells and the serum above it, so the serum can be tested or transferred without being mixed back into the cells. This page is for lab staff and supply managers who need to tell SSTs apart from other tubes, read the label correctly and understand where the tube sits in standards and the order of draw.
What is a serum separator tube?
An SST is a tube in which blood is allowed to clot. The clotted portion is the cells plus fibrin; the liquid above it after centrifugation is serum. The gel is a physical barrier formed during centrifugation. In the United States, FDA lists these devices under product code JKA, “Tubes, vials, systems, serum separators, blood collection”, regulated under 21 CFR 862.1675 as a “Blood specimen collection device” in Class 2 (FDA product classification database, checked 2026-11-07).
Closure colour is a manufacturer convention, not a universal rule. CLSI’s order-of-draw guidance groups serum tubes “including those with clot activator and gels (eg, red, red-speckled, gold closures)” under one heading, which shows that more than one colour is used for serum tubes. For colour families see blood draw tube colors.
Serum vs plasma tubes: what is the difference?
Serum comes from blood that has been allowed to clot; plasma comes from blood that has been kept from clotting by an anticoagulant such as heparin or EDTA. Clotting converts fibrinogen to fibrin, which ends up in the clot rather than in the serum. Serum tubes therefore carry a clot activator (or none), and plasma tubes carry an anticoagulant. The two are not interchangeable for a given test; the testing laboratory specifies which one it needs. A fuller comparison is in serum vs plasma.
What is a fibrin clot in serum?
A fibrin clot is the mesh formed when fibrinogen is converted to fibrin during clotting; it traps the blood cells. In a correctly clotted and centrifuged serum tube the clot stays below the gel or at the bottom of the tube. Clotting and spin conditions for a given tube are set in the manufacturer’s IFU.
SST and related tube types
| Tube category (CLSI order-of-draw grouping) | What it is | Specimen produced | Limits to know |
|---|---|---|---|
| Serum tube with clot activator and gel (e.g. red-speckled, gold closures) | The SST: clot activator plus separator gel | Serum | Plastic serum tubes with a clot activator may interfere with coagulation testing (CLSI) |
| Serum tube, clot activator, no gel | Clotted-blood tube without a barrier; see red-top tube additive | Serum | No gel barrier, so serum is usually transferred off the clot |
| Heparin tube, with or without gel | Anticoagulated tube; the gel version is covered in PST tube color | Plasma | Plasma, not serum; different test suitability |
| EDTA tube, with or without gel | Anticoagulated tube | Plasma (or whole blood) | Not a substitute for a serum tube |
Source for the groupings and the coagulation-interference statement: CLSI, “Order of blood draw: tubes and additives” (checked 2026-11-07). The specimen column reflects the clot versus anticoagulant distinction described above.
How serum and plasma are separated
Both are separated by centrifugation, which packs the cells toward the bottom of the tube. In an SST the gel moves between the cells and the liquid during the spin. BD’s product page for the Vacutainer Rapid Serum Tube also states, for its gel SST and PST tubes, that those tubes “should be spun at room temperature at a speed of 1000 to 1300 RCF for 10 minutes in a swinging bucket centrifuge” and “15 minutes in a fixed-angle centrifuge” (BD product page, checked 2026-11-07). The rotor type changes the time, which is why the setting should come from the IFU of the tube in hand, not from another brand. Rotor background is in what is a centrifuge.
What does a serum separator tube do?
It lets a clotted sample be spun once so serum sits above a barrier, reducing contact between serum and cells. The same BD page describes its gel SST and PST tubes as coated with silicone and micronized silica particles “to accelerate clotting”. Clot activator chemistry differs by product and manufacturer, so check the IFU of the tube in hand; BD’s separate non-gel Vacutainer Rapid Serum Tube, for example, is a different product with a thrombin-based activator and no separator gel.
How to choose
- Start with the test. The testing laboratory’s collection manual states whether serum, plasma or whole blood is required, and which tube it accepts. Colours vary by manufacturer, so colour alone is not a reliable guide.
- Confirm the specimen type. Serum needs a clotted-blood tube; plasma needs an anticoagulant tube.
- Check order of draw. CLSI places serum tubes after blood culture and sodium citrate, and before heparin, EDTA and fluoride/oxalate tubes. See order of draw for venipuncture. Before a coagulation tube, only a culture tube, a glass non-additive serum tube or a plastic serum tube without clot activator is listed ahead of it.
- Small or difficult draws. Capillary collection uses different containers; see capillary blood collection.
What to check on the label
- Additive and whether a gel is present (the product page, not the colour, is authoritative).
- Closure colour, noting it is manufacturer-specific (e.g. BD lists an orange Hemogard closure for one rapid-clot product).
- Tube size and draw volume as printed on the product specification (BD lists 13×100 for that product).
- Glass or plastic. The CLSI coagulation caveat applies to plastic serum tubes with a clot activator.
- Expiration date; BD notes it is printed on the tube label in Year/Month/Day form.
- Clotting time, inversion count and centrifuge settings in the IFU.
Care and replacement
Use tubes before the printed expiration date and keep storage to the conditions in the IFU for the product. Once a gel barrier has formed after centrifugation, whether re-spinning is allowed is a question for the IFU or the laboratory; do not assume it is.
FAQ
What is the difference between plasma and serum tubes?
Serum tubes let blood clot (clot activator, sometimes with gel); plasma tubes contain an anticoagulant such as heparin or EDTA so the blood does not clot.
What is a fibrin clot in serum?
Fibrin is formed from fibrinogen during clotting. It belongs in the clot, not in serum; it is not part of the serum layer.
How can we separate plasma and serum?
By centrifuging the collected tube; settings come from the tube’s IFU. Gel tubes keep the separated layer apart from the cells.
What does a serum separator tube do?
It combines a clot activator with a gel barrier so that serum can be separated from the clotted cells in a single centrifugation.
Sources
- FDA, Product Classification, product code JKA, 21 CFR 862.1675, Class 2: accessdata.fda.gov (checked 2026-11-07).
- CLSI, Order of blood draw: tubes and additives: clsi.org (checked 2026-11-07).
- BD, Vacutainer Rapid Serum Tube product page and FAQ: bd.com (checked 2026-11-07).
