Phlebotomy (CPT) guide · updated 2026-09-29

Order of draw: the one list every CPT candidate must know

For venipuncture, CLSI sets this order: blood cultures or sterile tubes, light blue (sodium citrate), red or gold serum tubes, green (heparin), lavender (EDTA), then gray (sodium fluoride and potassium oxalate). For capillary collection the order changes: EDTA first, then other additive tubes, then serum.

Why order matters

Additives can ride along on the needle into the next tube. A trace of EDTA carried into a chemistry tube binds calcium and adds potassium (EDTA tubes usually contain potassium EDTA), so a healthy patient can come back with a falsely high potassium and falsely low calcium. The order of draw puts the tubes most sensitive to contamination first and the tubes with the strongest additives last.

Venipuncture order (CLSI)

Inversion counts follow the major tube manufacturer and are the numbers most prep materials use. Always follow the manufacturer insert for the tubes in front of you.

Order	Tube	Additive	Common tests	Inversions
1	Blood culture bottles or sterile tubes	Broth medium (bottles)	Blood cultures	Per manufacturer
2	Light blue	Sodium citrate	PT/INR, aPTT and other coagulation tests	3 to 4
3	Red or gold (serum separator)	None or clot activator, with or without gel	Chemistry, serology, many send-outs	5 (plastic with clot activator)
4	Green	Lithium or sodium heparin	Plasma chemistry, STAT chemistry	8 to 10
5	Lavender or pink	K2 EDTA	CBC, HbA1c, blood bank (pink)	8 to 10
6	Gray	Sodium fluoride and potassium oxalate	Glucose, lactic acid, blood alcohol	8 to 10

Light blue: the tube with the most rules

It must be filled to the line because the test depends on a 9 to 1 ratio of blood to citrate.
When a winged (butterfly) set is used and the light blue tube is first, draw a discard tube first to fill the air in the tubing, otherwise the tube underfills.
Mix gently. Shaking causes hemolysis and can activate platelets.

Capillary order is different

In a skin puncture the blood is already starting to clot as it flows. Platelets clump quickly, so the EDTA microtube is collected first to protect the cell counts. After that come other additive tubes such as heparin, and serum tubes go last because they are supposed to clot anyway.

Wipe away the first drop of blood before collecting, since it tends to contain tissue fluid.

A way to remember it

Many students use the phrase Stop Light Red, Stay Put, Green Light Go, which maps word by word to sterile, light blue, red, SST (serum separator), PST (plasma separator, a green top with gel), green, lavender, gray. Pink EDTA tubes sit with lavender.

How the exam asks about it

Straight recall (which tube comes after light blue), reverse recall (which additive is in a gray top), and scenario items where a result looks wrong and you have to spot the order-of-draw mistake that caused it. The sample questions below are drawn from the Routine Blood Collections domain.

Sample Phlebotomy (CPT) questions with answers

1. A phlebotomist is ordered to collect blood cultures, a prothrombin time (PT/INR), and a CBC. According to the CLSI venipuncture order of draw, which sequence is correct?

Light blue → blood cultures → lavender
Blood cultures → light blue → lavender
Blood cultures → lavender → light blue
Lavender → blood cultures → light blue

Show answer

B. Blood cultures → light blue → lavender CLSI GP41 places sterile blood culture bottles first to prevent additive carryover from contaminating the sterile specimen, light blue sodium citrate second because the critical citrate-to-blood ratio must not be disrupted by carryover, and EDTA (lavender) last among these three. Options A and D do not place blood cultures first; option C reverses the light blue and lavender positions.

2. How many times should a lavender-top EDTA tube be inverted immediately after venipuncture collection, according to CLSI GP41?

3 to 4
5
6 to 7
8 to 10

Show answer

D. 8 to 10 CLSI GP41 requires 8 to 10 gentle inversions for EDTA tubes to ensure the anticoagulant is thoroughly mixed with the blood and to prevent microclot formation, which would falsely lower the platelet count and invalidate CBC results. Three to four inversions is the standard for sodium citrate tubes; 5 inversions applies to clot-activator SST tubes; 6 to 7 is not a CLSI-specified range for any standard tube type.

Frequently asked questions

Does the gold SST come before or after light blue?

After. Light blue sodium citrate is drawn before any serum tube, including red and gold serum separator tubes.

Why is EDTA drawn late in venipuncture but first in capillary collection?

In venipuncture EDTA goes late to avoid contaminating other tubes. In capillary collection it goes first because platelets clump quickly at a skin puncture and the cell counts need the freshest blood.

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Sources: NHA CPT Test Plan (2024 job analysis, exam live January 7, 2026) · CLSI: Order of blood draw tubes and additives. Independent study material. Not affiliated with or endorsed by the National Healthcareer Association.

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NHA and National Healthcareer Association are trademarks of the National Healthcareer Association. ExamWeight is an independent study resource and is not affiliated with, endorsed by, or sponsored by NHA. Questions are original and written against the published Certified Phlebotomy Technician (CPT) test plan.