Guide
Order of draw: tube colours, additives, and the WHO sequence
6-minute read · Built on WHO Best Practices in Phlebotomy (2010)
When more than one blood tube is drawn from a single venepuncture, the order in which the tubes are filled matters. A small amount of additive can be carried from one tube into the next on the needle, and that invisible carryover can change a result. Drawing the tubes in the right sequence is how a phlebotomist keeps every sample valid.
Why the order of draw matters
Each collection tube carries an additive — a substance that prevents clotting, accelerates it, or preserves a particular analyte. When the rubber-sleeved end of the needle passes from one tube to the next, a trace of additive can travel with it. This is additive carryover, and it is the whole reason the order of draw exists.
The classic example: EDTA tracked from a purple-top tube into a chemistry tube binds calcium and releases potassium, so the laboratory reports a falsely low calcium and a falsely high potassium. Drawing the tubes in a fixed sequence keeps the additives that matter most away from the tests they would distort.
The WHO order of draw
The condensed working sequence most laboratories teach, based on WHO Best Practices in Phlebotomy (Table 2.3, drawn from the CLSI consensus), is:
| Order | Tube (common colour) | Additive | Typical tests |
|---|---|---|---|
| 1 | Blood culture (yellow-black) | Broth medium | Microbiology cultures |
| 2 | Sodium citrate (light blue) | Sodium citrate | Coagulation (PT, APTT, INR) |
| 3 | Serum (red), then SST (gold/tiger) | Clot activator ± gel | Biochemistry, serology |
| 4 | Heparin (dark green / light green) | Heparin | Stat plasma chemistry |
| 5 | EDTA (purple / lavender) | EDTA | Full blood count, cross-match |
| 6 | Fluoride-oxalate (grey) | Fluoride-oxalate | Glucose, lactate |
Blood cultures go first so the culture set is collected before any other tube can introduce skin flora. Citrate comes early because coagulation results are the most sensitive to ratio error. EDTA sits late because it is the additive most likely to contaminate the chemistry tubes that precede it.
WHO BPP 2010Draw blood collection tubes in the correct order, to avoid cross-contamination of additives between tubes. As colour coding and tube additives may vary, verify recommendations with local laboratories.
— WHO Best Practices in Phlebotomy 2010, 2.2.3
Two cases worth knowing
A coagulation assay drawn on its own does not need a preceding discard tube — a single light-blue citrate tube may be drawn alone (WHO Table 2.3, footnote d).
When a winged (butterfly) set is used, the air in its tubing — the dead space — under-fills the first tube. If a citrate tube would otherwise be first, draw a small discard tube first so the citrate tube fills completely and its 1:9 ratio stays correct.
This guide is a free extract from PhlebMastery's WHO-based phlebotomy theory course. The full treatment — tube additives, gauge selection, and the equipment behind a valid sample — 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.