3.3 Tubes, Anticoagulants, and Order of Draw
Key Takeaways
- Follow CLSI order: blood cultures → light-blue citrate → serum (red/gold SST) → green heparin → lavender/pink EDTA → gray fluoride/oxalate (specialty tubes per facility)
- Light-blue citrate requires a full 9:1 blood-to-additive fill; underfill falsely prolongs clot-based coag results
- Invert additive tubes gently the recommended number of times; never shake; EDTA or heparin carryover ruins later chemistry and coag results
- Keep tourniquet time ≤1 minute; use discard tubes with butterflies before light-blue when air in tubing would underfill citrate
3.3 Tubes, Anticoagulants, and Order of Draw
Quick Answer: Draw in CLSI order—blood culture → light blue (citrate, full 9:1) → serum red/gold → green heparin → lavender/pink EDTA → gray fluoride/oxalate. Mix with correct gentle inversions. Prevent additive carryover. Limit tourniquet to ≤1 minute. Use a discard tube before light-blue draws on butterflies when tubing air would underfill citrate.
If identification is the first law of phlebotomy, order of draw is the second. Tubes contain powerful chemicals. Drawing them in the wrong sequence lets additives hitchhike on the needle into the next tube and silently destroy result integrity.
Why Order of Draw Exists
Carryover happens when residual additive on the needle tip or holder contaminates the following tube. Classic disasters:
- EDTA before chemistry serum/heparin tubes: potassium skyrockets (EDTA is often K2/K3 salt) and calcium falls (EDTA chelates Ca²⁺).
- Heparin or EDTA before citrate coag tubes: PT/aPTT can be falsely prolonged.
- Clot activator or gel issues when order and mixing are sloppy can impair other specialty assays.
CLSI’s recommended sequence standardizes protection against these errors across evacuated tube systems.
Full CLSI-Oriented Order of Draw (Teaching Sequence)
Use this core sequence for MLA study (facility charts may add specialty tubes such as ACD yellow, trace-element royal blue, or thrombin tubes in defined slots):
| Order | Tube / bottle | Common additive | Primary purpose |
|---|---|---|---|
| 1 | Blood culture bottles (or sterile SPS) | SPS / culture media | Microbiology—draw first to protect sterility |
| 2 | Light blue | 3.2% sodium citrate | Coagulation (PT/INR, aPTT, etc.)—9:1 fill |
| 3 | Serum: red, gold, tiger SST | Clot activator ± gel | Chemistry, serology, many immunoassays |
| 4 | Green / light green PST | Lithium or sodium heparin ± gel | Plasma chemistry, many STAT panels |
| 5 | Lavender / purple and pink | EDTA | CBC, HbA1c, blood bank (pink often preferred for T&S) |
| 6 | Gray | Sodium fluoride + potassium oxalate (typical) | Glucose preservation, lactate (per protocol), alcohol (policy) |
Memory scaffold
Cultures → Citrate → Serum → Heparin → EDTA → Fluoride
(“Sterile start, blue clotting times, then chemistry colors, purple cells, gray sugar.”)
Always verify your laboratory’s wall chart—especially for royal blue, yellow ACD, and black ESR tubes—but the six-step core above is exam-essential.
Additive Function Table
| Additive | Tube colors (typical) | Mechanism | What fails if misused |
|---|---|---|---|
| SPS | Blood culture / yellow sterile | Anticoagulates and supports microbes | Contaminated cultures if drawn late or with poor prep |
| Sodium citrate | Light blue | Reversible Ca²⁺ chelation; preserves coag factors for testing | Underfill → excess citrate → falsely long PT/aPTT; overfill can shorten |
| Clot activator (silica) | Plastic red, gold SST | Speeds clotting | Incomplete mix → delayed clot, fibrin strands in serum |
| Separator gel | Gold SST, light green PST | Barrier after centrifuge | Rough handling/re-spin issues per lab rules |
| Heparin | Green | Potentiates antithrombin → inhibits thrombin | Wrong heparin salt can affect some analytes (e.g., Li heparin vs Li measurement); microclots if not mixed |
| EDTA | Lavender, pink | Strong Ca²⁺ chelation; preserves cell morphology | Clots if under-mixed; carryover wrecks K⁺/Ca²⁺ on chemistry |
| NaF + oxalate | Gray | Fluoride inhibits glycolysis; oxalate anticoagulates | Underfill/poor mix; not a universal chemistry tube |
Whole blood vs plasma vs serum (preview for 3.4):
- Serum = clotted blood, no anticoagulant (red/SST).
- Plasma = anticoagulated, cells removed (citrate, heparin, EDTA plasma).
- Whole blood = anticoagulated, not separated (CBC in EDTA).
Inversion Counts (Gentle Mixing)
One inversion = tip tube fully upside down and back so the bubble travels the length of the tube. Never shake.
| Tube | Typical gentle inversions |
|---|---|
| Light blue (citrate) | 3–4 |
| Serum with clot activator (plastic red/SST) | 5 |
| Green heparin | 8–10 |
| Lavender/pink EDTA | 8–10 |
| Gray fluoride/oxalate | 8–10 |
| Glass red, no additive | 0 |
Under-mixing → microclots in EDTA/heparin (platelet clumps, false CBC flags) or incomplete clotting in SST. Over-vigorous mixing → hemolysis.
Fill Volume: Underfill and Overfill
Evacuated tubes are engineered for a target draw volume printed on the label.
Light blue citrate — the critical 9:1 ratio
Blood : citrate should be 9:1. The vacuum and liquid citrate volume are matched so a full draw hits the ratio.
- Underfill (short draw): too much citrate relative to blood → excess calcium binding when the test reagent adds calcium → falsely prolonged PT/INR and aPTT. Labs usually reject underfilled coag tubes.
- Overfill: relatively less citrate → possible falsely shortened times or clots.
Other tubes
- EDTA underfill: excess EDTA shrinks cells → falsely low Hct/MCV patterns and morphology artifacts; possible clotting if vacuum failed entirely.
- EDTA overfill / under-mixed: clots → rejected CBC.
- Heparin underfill: clotting risk, platelet clumps.
- Gray tubes: incorrect fill compromises glucose stability claims.
If vacuum is lost (expired tube, needle not fully engaged), replace the tube—do not “top off” from another tube or syringe into an additive tube without validated transfer devices and policy (and never pour tube-to-tube).
Tourniquet ≤1 Minute (Reinforced)
Prolonged tourniquet time causes hemoconcentration: water leaves the vein, concentrating cells and large molecules. Potassium, proteins, enzymes, and cell counts can shift. Hemolysis risk also rises with difficult, long searches under a tight band.
Practice: release as soon as blood flows steadily, or at 1 minute—whichever comes first. If you must repalpate after lengthy prep, release during cleansing and reapply briefly.
Discard Tubes and Butterfly (Winged Set) Considerations
Butterfly tubing contains air. When the first tube is a light-blue citrate, that air can prevent a full draw, destroying the 9:1 ratio.
Rule taught on exams and used in many labs:
If a citrate tube is first (or first additive tube) with a butterfly, draw a discard tube first (non-additive or appropriate discard) to clear air, then fill the light blue completely. If blood cultures are ordered, they still come first in order; follow facility SOP for culture-plus-coag sequences with winged sets.
Discard tubes are also used when drawing from vascular access devices after a waste volume, which is typically a nursing/line-draw procedure.
Blood Culture Technique Notes (Order Position 1)
- Prep skin with the facility blood-culture antiseptic (often chlorhexidine; iodine still used in some protocols)—not a casual quick alcohol wipe.
- Do not repalpate after sterile prep unless sterile gloves/technique allow.
- Fill aerobic/anaerobic bottles to marked volumes; underfill reduces recovery, overfill can cause false signals on some instruments.
- Draw cultures before anticoagulant tubes to minimize contamination from nonsterile stoppers and to prioritize sterile collection.
Putting It Together: Multi-Tube Draw Example
Order: blood cultures, PT/INR, CMP, CBC, fasting glucose (gray).
- Identify patient; assess arm; tourniquet briefly; cleanse culture site per protocol.
- Blood culture bottles first.
- Light blue—full fill; 3–4 inversions (discard first if butterfly air risk).
- Gold SST for CMP; 5 inversions; allow clot time before centrifuge.
- Lavender EDTA for CBC; 8–10 inversions.
- Gray for glucose; 8–10 inversions.
- Release tourniquet ≤1 minute into the process; label at bedside; transport.
Wrong order example that fails quality: lavender before gold → EDTA carryover → panic-level false hyperkalemia and hypocalcemia on CMP.
Troubleshooting Table
| Problem | Likely cause | Prevention |
|---|---|---|
| Rejected PT, long clotting times | Underfilled blue; heparin carryover; hard stick/hemolysis | Full fill; correct order; clean draw |
| Critical high K⁺, low Ca²⁺ | EDTA contamination of chemistry tube | EDTA after serum/heparin; never pour EDTA blood into SST |
| Clotted CBC | No/poor EDTA mix; difficult draw | Immediate 8–10 inversions |
| Hemolyzed specimen | Tiny needle + large vacuum, shaking, wet alcohol, traumatic stick | Match gauge/tube; gentle mix; dry site |
| Short blue on butterfly | Air in tubing | Discard tube, then blue |
Key Takeaways
- Memorize culture → blue → serum → green → lavender/pink → gray.
- Citrate 9:1 is non-negotiable; underfill invalidates coag.
- Inversions mix without hemolysis; carryover is why order exists.
- Tourniquet ≤1 minute; discard before light blue on butterflies when needed.
According to the standard CLSI-oriented order of draw, which tube is collected immediately after blood culture bottles?
Why must a light-blue coagulation tube be filled to the proper line (approximately 9:1 blood-to-citrate)?
Drawing a lavender EDTA tube before a serum chemistry tube is most likely to cause which carryover pattern?
When using a butterfly set and the first tube needed is light-blue citrate, what special step is often required?