Collection and order of draw — Clinical Laboratory Practice

What you'll learn

Recall the order of draw
Explain how the order of draw prevents additive carry-over
Recognise the result pattern of EDTA contamination
Describe how tubes are filled, mixed, labelled and transported after the draw

Recall the standard order of draw, why it prevents additive carry-over, and how to handle tubes after collection.

Why it matters

Blood collection tubes contain different additives: anticoagulants, clot activators, gels and preservatives. When several tubes are filled from one needle, a tiny amount of additive can be carried from one tube into the next. If EDTA from a lavender tube gets into a chemistry tube, it binds calcium and adds potassium, so the report may show a dangerously high potassium and a low calcium that the patient does not really have. The order of draw exists to prevent that carry-over.

Safety and scope. Venipuncture is a clinical skill learned in an accredited phlebotomy, MLT or MLS program and practiced only under supervision on consenting patients, after bloodborne-pathogen and sharps training. Never practice on yourself or anyone else at home. This lesson is knowledge only; nothing here is medical advice.

The core idea: the standard order

The order below follows the CLSI collection standard used by most US labs (always check your own lab's procedure, which is the version you follow):

Order	Typical cap color	Additive	Common uses
1	Blood culture bottles	Culture medium	Blood cultures (drawn first to stay sterile)
2	Light blue	Sodium citrate	Coagulation (PT/INR, aPTT)
3	Red or gold	None, or clot activator with or without gel	Serum chemistry, serology
4	Green	Heparin (lithium or sodium)	Plasma chemistry
5	Lavender or pink	EDTA	CBC, HbA1c, blood bank
6	Grey	Sodium fluoride / potassium oxalate	Glucose, lactate

Cap colors are conventions and can vary by manufacturer and country, so the label on the tube (the additive name) is what counts.

Why this order works

Cultures first, because sterility matters most and nothing should touch the needle before them.
Citrate early, because coagulation tests are sensitive to clot activators and to tissue factor. If a butterfly set is used and citrate is the first tube, a discard tube is drawn first to fill the tubing's dead space, so the citrate tube still fills to the correct 9:1 ratio.
Serum tubes before plasma tubes, so clot activator is not carried into anticoagulated tubes.
EDTA and fluoride/oxalate last, because their contamination does the most damage: EDTA (as potassium EDTA) raises potassium and binds calcium and magnesium; oxalate and fluoride also bind calcium and inhibit enzymes.

Handling after the draw

Order is only half the job. Each tube must also be:

Filled correctly. Vacuum tubes are designed to draw the right volume for the additive.
Inverted gently the number of times the manufacturer specifies (often 3 to 4 for citrate and 5 to 10 for others). Shaking causes hemolysis; not mixing lets anticoagulated tubes clot.
Labelled at the patient's side immediately after collection (see the next lesson).
Transported within the time and temperature limits for each test.

Worked example: planning a draw

A requisition (fictional) asks for blood cultures, PT/INR, a basic metabolic panel in a gold tube, a CBC and a glucose in a grey tube. Using a butterfly set, the plan is:

Blood culture bottles (aerobic, then anaerobic, per the manufacturer).
Light blue (citrate). A discard tube is not needed here, because the culture bottles already filled the tubing; if cultures were not ordered, a discard tube would be drawn first.
Gold (serum, clot activator and gel).
Lavender (EDTA).
Grey (fluoride/oxalate).

If the lab later sees a potassium of 9.5 mmol/L with a calcium near zero in the gold tube (a combination incompatible with life in a patient who is talking), EDTA contamination is the first suspect. The lab would reject the result and request a redraw, not call it as a critical value.

A memory aid

Many students remember the sequence as "Stop, Light, Red, Stay Put, Green Light, Go": Sterile (cultures), Light blue, Red, SST (gold), Green, Lavender, Grey. Any mnemonic is fine as long as you can explain why each tube sits where it does; the reasons are what help you troubleshoot.

Common mistakes

Drawing the EDTA tube before chemistry tubes, causing false high potassium and false low calcium.
Forgetting the discard tube when citrate is first with a butterfly set, so the citrate tube is under-filled.
Trusting cap color instead of reading the additive on the label.
Shaking tubes vigorously instead of gentle inversion.

Quick recap

Order of draw prevents additive carry-over: cultures, citrate, serum, heparin, EDTA, fluoride/oxalate.
EDTA contamination looks like very high potassium with very low calcium.
Fill, invert gently, label at the patient's side, and transport on time.
Phlebotomy itself is learned and practiced only in supervised clinical training.

Try it

Open the Do tab for the quiz: sequence a draw and diagnose a contaminated result.

Key terms (5)

order of draw: The standard sequence for filling tubes from one needle so additives are not carried into the next tube.
additive: A substance in a collection tube, such as an anticoagulant, clot activator, gel or preservative.
carry-over: A tiny amount of one tube's additive getting into the next tube filled from the same needle.
EDTA: The anticoagulant in lavender tubes; as potassium EDTA it raises potassium and binds calcium if it contaminates a chemistry tube.
discard tube: A tube drawn first with a butterfly set to fill the tubing's dead space so a following citrate tube fills to the correct ratio.