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The Phlebotomy Order of Draw, in Six Steps.

The CLSI H3-A6 sequence exists for one reason: additives carry between tubes. A drop of EDTA in a chemistry tube is enough to flip a normal calcium into a critical low. Memorize the order, then memorize the why.

01 · The sequence

CLSI H3-A6 order of draw chart.

  1. 1
    Blood culture
    Yellow SPS bottle
    Sterile site first. Always.
  2. 2
    Light Blue
    Sodium citrate (coagulation)
    Protected from any tissue thromboplastin.
  3. 3
    Red / Gold
    Serum group (no anticoagulant)
    Either order within the group is acceptable.
  4. 4
    Green
    Heparin (PST)
    Plasma chemistry, before any chelating additives.
  5. 5
    Lavender / Pink
    EDTA
    EDTA carryover ruins downstream chemistry, so it sits late.
  6. 6
    Gray
    Fluoride / oxalate
    LAST. No exceptions. Fluoride and oxalate contaminate anything earlier.
02 · Mnemonics

Two ways to remember the sequence.

The CLSI version is the one your educator will accept. The informal version is the one most students actually remember at 6 a.m. on a clinical rotation. Use whichever sticks.

CLSI standard

Stop Light Red Stay Put Green Light Go

S · L · R · S · P · G · L · G
Informal

Boys Love Ravishing Girls Like Sexy Goddesses

B · L · R · G · L · S · G
03 · What goes wrong

Three carryover errors that show up on every exam.

EDTA into chemistry

Draw the lavender before the green or gold and a few microliters of EDTA reach the chemistry tube. EDTA chelates calcium and binds magnesium. Calcium drops, potassium spikes, and the patient looks like they are crashing when nothing is wrong.

Heparin into coagulation

Draw a green tube before the light blue and heparin contaminates the citrate tube. Heparin is itself an anticoagulant that prolongs PT and aPTT. Result: a normal patient looks like they are over-anticoagulated. The coagulation result is invalid.

Fluoride backwash

A gray top drawn before the lavender contaminates the EDTA tube with sodium fluoride. Fluoride inhibits enzymes used in the CBC analyzer. The CBC errors out, the gray glucose looks fine, and the recollect order is on you.

04 · Exam style

Three sample questions you will see on the NHA CPT.

  1. 1. A phlebotomist draws a CBC, a CMP, and a PT. Using a butterfly, in what order should the tubes be collected?

    Show answer

    Discard tube first, then light blue (PT), then gold/SST (CMP), then lavender (CBC). The discard tube fills the butterfly dead space so the citrate tube fills to the line. After that the order is 2, 3, 5 of the CLSI sequence.

  2. 2. A patient has a critical-low calcium of 1.1 mmol/L on a chemistry panel. Repeat draw is normal. What is the most likely cause of the original result?

    Show answer

    EDTA carryover. The lavender tube was drawn before the chemistry tube and a small amount of EDTA chelated the calcium in the chemistry sample. Drawing in CLSI order would have prevented this.

  3. 3. Which tube goes last in every draw, with no exceptions?

    Show answer

    Gray (sodium fluoride and potassium oxalate). Both additives contaminate any earlier tube they reach.

05 · Tubes in the sequence

The seven tubes by CLSI position.

06 · Questions

Order-of-draw FAQs.

What is the order of draw in phlebotomy?
The CLSI-recommended order is: (1) blood culture (yellow SPS), (2) light blue (sodium citrate), (3) red and gold (serum group, either order), (4) green (heparin), (5) lavender (EDTA), (6) gray (fluoride/oxalate). The sequence prevents additive carryover between tubes.
What is the easiest order of draw mnemonic?
The most commonly taught is "Stop Light Red Stay Put Green Light Go" mapping to Sterile, Light blue, Red, Serum gold, Plasma green, Lavender, Glucose gray. An informal version some students remember more easily is "Boys Love Ravishing Girls Like Sexy Goddesses."
Why does the order of draw matter?
Tiny amounts of additive transfer from one tube to the next through the needle hub or stopper. EDTA carryover into a chemistry tube falsely lowers calcium and elevates potassium. Heparin carryover into a coagulation tube prolongs PT and PTT. The sequence puts each additive after every tube it could ruin.
Which tube is drawn first?
Blood culture bottles when ordered, because they require sterile technique. If no cultures are ordered, the light blue (sodium citrate) tube is first. Drawing it first means there is no risk of additive carryover into the coagulation result.
Why is the gray tube last?
Gray contains sodium fluoride and potassium oxalate. Both will contaminate any earlier tube if they carry over. Fluoride inhibits enzymes used in chemistry analyzers and oxalate damages cell membranes. Drawing gray last means its additives never touch another tube.
Do red and gold (SST) tubes have a fixed order between them?
No. Both are serum tubes and either order within the group is acceptable per CLSI. In practice, plain red is usually drawn before SST when both are needed because plain red has zero additive risk. Some labs standardize one order for consistency.
What about the discard tube rule for butterfly draws?
When a butterfly (winged infusion set) is used and the light blue is the first tube, a discard tube must be drawn first. Air in the butterfly tubing reduces the citrate fill volume and breaks the 9:1 ratio. The discard tube fills the dead space so the citrate tube fills correctly.
Where does the royal blue tube go in the order of draw?
It depends on the additive. EDTA royal blue follows lavender logic and falls at position 5. Plain royal blue is treated as a no-additive serum tube and falls in position 3 with red and gold. The cap color alone does not tell you the position. Confirm the additive before drawing.
07 · Free practice

Order of draw shows up on every NHA CPT exam.

PhleboPrep gives you 10 free practice questions every week, with the same scenario format you will see on test day.

Try free practice →