Blood Collection Tube Colors and Additives
Updated October 27, 2026
Each blood collection tube’s stopper color tells you what additive it contains, and the additive decides which tests the tube can be used for. Knowing tube colors and additives is basic to the NHA CPT exam. Here’s a reference chart, followed by free practice questions. Tube colors are standardized for most manufacturers, but always check the tube label and your laboratory’s test directory.
Tube colors and additives
| Stopper color | Additive | What it does | Common tests |
|---|---|---|---|
| Light blue | Sodium citrate | Anticoagulant: binds calcium | Coagulation (PT/INR, PTT) |
| Red | None (glass) or clot activator (plastic) | Lets blood clot to produce serum | Chemistry, serology, blood bank in some labs |
| Gold or red/gray (SST) | Clot activator and gel separator | Gel separates serum from cells after centrifuging | Chemistry |
| Green | Heparin (lithium or sodium) | Anticoagulant: stops thrombin | Plasma chemistry |
| Light green (PST) | Lithium heparin and gel | Gel separates plasma from cells | Plasma chemistry |
| Lavender or purple | EDTA | Anticoagulant: binds calcium | Hematology (CBC), HbA1c |
| Pink | EDTA | Anticoagulant | Blood bank (type and screen) |
| Gray | Sodium fluoride and potassium oxalate | Fluoride stops glucose breakdown; oxalate is an anticoagulant | Glucose, lactic acid, alcohol |
Serum versus plasma
- Serum comes from blood that has clotted (red, gold tubes).
- Plasma comes from blood that didn’t clot because of an anticoagulant (light blue, green, lavender tubes).
Inversions
Gently invert each tube right after filling, as the manufacturer directs. Common counts: 3 to 4 for light blue, 5 for clot activator and serum separator tubes, and 8 to 10 for green, lavender and gray tubes.
See our order of draw for the sequence.
Free practice questions
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Which additive is in a light blue tube?Answer: BLight blue tubes contain sodium citrate, which binds calcium to prevent clotting, for coagulation tests.
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Which tube is usually used for a complete blood count (CBC)?Answer: AEDTA preserves the shape of blood cells, so lavender tubes are used for hematology tests such as the CBC.
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Why does a gray-top tube contain sodium fluoride?Answer: CSodium fluoride stops glycolysis, so glucose levels don't drop while the sample waits to be tested.
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What is the difference between serum and plasma?Answer: DSerum is the liquid left after blood clots. Plasma is the liquid part of blood that was kept from clotting.
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What is the gel in an SST (gold) tube for?Answer: BThe gel forms a barrier between the serum and the cells when the tube is centrifuged.
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How many times is a lavender tube typically inverted?Answer: DLavender (EDTA) tubes are typically inverted 8 to 10 times to mix the anticoagulant with the blood. Follow the manufacturer's instructions.
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