Guide
Tube colours and additives: what each blood collection tube does
6-minute read · Built on WHO Best Practices in Phlebotomy (2010)
When a phlebotomist reaches for a blood tube, the colour of the stopper is not decoration. It tells you the additive inside, and the additive decides what the laboratory can do with the sample. Choose the wrong colour and the result can look perfectly normal while being clinically wrong.
The colour is the additive
Every collection tube is built for a class of test, and most carry an additive — a substance that changes what happens to the blood between the draw and the analysis. Some additives stop the blood clotting; one accelerates it; others preserve a single analyte or help the laboratory separate serum or plasma during centrifugation. The stopper colour is the shorthand for which additive is inside.
| Tube (common colour) | Additive | What the additive does | Typical tests |
|---|---|---|---|
| Blood culture (yellow) | Broth medium | Keeps micro-organisms viable | Microbiology cultures |
| Light blue | Sodium citrate | Binds calcium in a fixed 1:9 ratio | Coagulation (PT, APTT, INR) |
| Red / gold | Clot activator | Lets blood clot so serum can separate | Biochemistry, serology |
| Green | Heparin | Inactivates thrombin to keep plasma | Stat plasma chemistry |
| Purple / lavender | EDTA | Binds calcium to preserve cell shape | Full blood count, cross-match |
| Grey | Fluoride-oxalate | Slows glycolysis to protect glucose | Glucose, lactate |
EDTA, citrate, and heparin are all anticoagulants, but each works by a different mechanism and serves a different class of test — which is why they are not interchangeable.
Fill the tube to the line
An additive tube only works at the right ratio of blood to additive. The sodium citrate coagulation tube is the strict case: it is calibrated for a 1:9 ratio of citrate to blood, so an under-filled tube holds too much citrate for the blood present, and the coagulation result no longer reflects the patient. The laboratory rejects it. EDTA has a similar but more forgiving dependency; heparin tubes tolerate a partial fill better than either.
Mix by inversion, never by shaking
Once filled, an additive tube needs mixing so the additive reaches all of the blood — but the mixing has to be gentle. An inversion is a slow top-to-bottom turn and back, repeated the manufacturer-specified number of times. Shaking generates the mechanical force that ruptures red cells, so the act meant to preserve the sample causes haemolysis instead.
When several tubes are drawn together, the colour also sets the sequence — see the order of draw, which stops additive carryover from one tube contaminating the next.
This guide is a free extract from PhlebMastery's WHO-based phlebotomy theory course. The full treatment — every tube, the equipment behind the draw, and the order of draw — is in Module 3: Equipment & Blood Collection Systems. New here? Start with the free Module 1, or see the whole course — full access is a one-time purchase.
Want the full picture? Read Module 3 in the course, or browse the glossary.