In short
- The CLSI order: blood culture, citrate, serum, heparin, EDTA, fluoride.[1]
- The order stops additives carrying over between tubes on the needle.[1]
- EDTA carryover can make potassium read high and calcium low. One UK study found it behind 14.3% of the low calcium results it investigated.[2]
- Cap colours vary between manufacturers, so read the label and follow your laboratory's chart.
What is the order of draw?
The order of draw is the sequence tubes are filled in when several are taken from one needle. The Clinical and Laboratory Standards Institute, in its standard GP41, gives this order:[1]
| Order | Tube | Usual cap | Additive | Commonly used for |
|---|---|---|---|---|
| 1 | Blood culture | Culture bottles | Culture medium | Detecting infection in the blood |
| 2 | Sodium citrate | Light blue | Sodium citrate | Clotting tests |
| 3 | Serum | Red or gold | None, or clot activator, with or without gel | Chemistry, hormones and serology |
| 4 | Heparin | Green | Heparin, with or without gel | Plasma chemistry |
| 5 | EDTA | Lavender | EDTA | Full blood count and HbA1c |
| 6 | Fluoride oxalate | Grey | Sodium fluoride and potassium oxalate | Glucose |
The "commonly used for" column is a rough guide. Your laboratory decides which tube each test needs, so follow its collection guide.
Why does the order of draw matter?
Each tube holds an additive, and a trace of it can travel on the needle into the next tube. The order is designed so that any carryover can't change the results of the tubes that follow.[1]
It's also why citrate comes so early: a plastic serum tube with clot activator can interfere with clotting tests, so only a blood culture, a glass serum tube without additive, or a plastic serum tube without clot activator should be drawn before a citrate tube.[1]
What happens if EDTA gets into another tube?
EDTA tubes contain potassium EDTA, which binds calcium. If it gets into a serum or heparin sample, potassium can read falsely high and calcium falsely low, results that could send a patient for treatment they don't need.[2]
It isn't rare. A UK hospital study found EDTA contamination behind 14.3% of the low calcium results it investigated, and when patients were bled again, 92% of their results came back normal.[2]
Why must the citrate tube be filled properly?
Clotting tests rely on the right ratio of blood to citrate, so an underfilled tube gives misleading results. With a winged (butterfly) set, air in the tubing can stop the first tube filling properly, which is why many laboratories ask for a discard tube before citrate. Follow your lab's rule.
Do cap colours mean the same thing on every brand?
Not always. The colours above are the common scheme[1], but some manufacturers use different colours for the same additive. Always read the tube label, and keep your laboratory's own chart where you take blood.
Where Drawn's colours come from
Drawn's colours are the caps of these tubes, in the order of draw: citrate blue, gold, green, lavender and grey. None of them is red. Read more about Drawn.
Sources
- [1]Order of blood draw tubes and additives, Clinical and Laboratory Standards Institute
- [2]EDTA sample contamination is common and often undetected, putting patients at unnecessary risk of harm, Sharratt and others, International Journal of Clinical Practice, 2009
This guide is general information for trained staff. It isn't a substitute for your laboratory's collection guide or your local policy. Facts checked 4 November 2026 against the sources listed.