Red Top Tube
None (glass) or clot activator (plastic)
The red top is the simplest tube in the rack. No additive (in true glass tubes) or just a clot activator (in plastic). Blood goes in, blood clots, you spin, you get serum. The catch is that not every red tube is the same, and the difference between glass and plastic determines the inversion count.
The additive.
A glass red top tube is true plain. No additive at all. Blood clots on its own from contact with the glass surface and the natural coagulation cascade. Inversions: zero. Add anything to the tube and you compromise the result.
A plastic red top contains a clot activator, usually micronized silica. The silica accelerates clot formation so the tube is ready to spin in 30 minutes instead of 60. Inversions: 5, gentle, to mix the silica into the blood.
Some manufacturers stripe red and gold caps together (the "tiger top") to indicate a clot activator and gel, but that is technically an SST/gold tube despite the red bands. Read the label, not just the cap stripe.
What the red top is used for.
Serology and immunology. Antibodies and antigen panels run on serum, not plasma. The red top gives clean serum without the gel barrier from an SST.
Blood bank crossmatch and antibody screening at many institutions. Blood bank prefers red or pink over SST because the gel can interfere with antigen-antibody reactions.
Drug levels for some specific drugs where the SST gel can absorb the drug into the gel layer. Therapeutic drug monitoring for certain medications still uses plain red.
How to draw it correctly.
Draw 6 to 10 mL. Red tubes are typically larger because serum yield matters and you cannot recover what is bound up in a clot.
Glass tubes: zero inversions. Plastic tubes: 5 gentle inversions. Get this wrong and the lab knows. Over-mixing a glass red can cause hemolysis; under-mixing a plastic red leaves silica unmixed.
Allow 30 to 60 minutes for the clot to form before centrifugation. Spin too early and you get fibrin strands in the serum, which clog analyzer probes.
Three mistakes that cost you a recollect.
- 1
Mixing up glass and plastic
Inverting a glass red causes hemolysis; not inverting a plastic red leaves the activator unmixed and the clot incomplete.
- 2
Spinning before the clot forms
Fibrin in the serum clogs analyzer probes. Wait 30 minutes for plastic, 60 minutes for glass.
- 3
Using red when blood bank wants pink
Some institutions require pink for crossmatch even when red would work. Check the protocol before drawing.
Red in the order of draw.
Red and gold (SST) are both serum tubes and share CLSI position 3. Either order within the group is acceptable per CLSI. In practice, plain red is often drawn before SST when both are needed because plain red has zero additive risk; some labs standardize the order for consistency.
See the full CLSI order-of-draw page →Common questions about the red top.
What is a red top tube used for?
How many times do I invert a red top tube?
Red top vs SST, when do I use which?
Why does the lab care about glass vs plastic red?
Six more tubes you will see on the same tray.
- Lavender Lavender/ PurpleCLSI 5 · EDTA (K2 or K3)
- Light Blue Light BlueCLSI 2 · Sodium citrate 3.2%
- Gold Gold/ SSTCLSI 3 · Clot activator + thixotropic gel
- Green GreenCLSI 4 · Lithium or sodium heparin (PST has gel)
- Gray GrayCLSI 6 · Sodium fluoride + potassium oxalate
- Royal Blue Royal BlueCLSI varies · Trace-element-free (EDTA or no additive)