Teach me order of blood draw and the rationale
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Here is a thorough breakdown of the order of blood draw, the additive in each tube, and exactly why that order matters.
Order of Blood Draw - Complete Guide
The order of draw is standardized by CLSI (Clinical and Laboratory Standards Institute) guideline GP41 to prevent cross-contamination of tube additives, which can cause false laboratory results.
The CLSI Order of Draw (Mnemonic: "Silly Boys Read Good Books Daily")
| Step | Tube Color | Additive | Common Tests |
|---|---|---|---|
| 1 | Yellow / Blood culture bottles | SPS (sodium polyanethol sulfonate) or none (sterile) | Blood cultures, sterile specimens |
| 2 | Light Blue | Sodium citrate (3.2%) | PT/INR, aPTT, D-dimer, fibrinogen, thrombin time |
| 3 | Red / Gold (SST) | None (red) or clot activator + gel (gold/SST) | CMP, LFTs, lipids, thyroid, most chemistries |
| 4 | Green | Sodium heparin or lithium heparin ± gel (PST) | Stat chemistries, ammonia, chromosomes |
| 5 | Lavender / Purple | EDTA (K2 or K3) | CBC, HbA1c, blood bank, lead, reticulocytes |
| 6 | Pink | EDTA (K2) | Blood bank / type & screen |
| 7 | Grey | Sodium fluoride + potassium oxalate | Glucose, lactate, alcohol |
| 8 | Yellow (ACD) | Acid citrate dextrose | HLA typing, paternity, DNA studies |
| 9 | Tan | K2EDTA (lead-free stopper) | Lead levels, trace metals |
| 10 | Royal Blue | EDTA or none | Trace metals, toxicology |
Mnemonic options: "Stop Looking Right, Have Enough Gray" - Sterile, Light blue, Red/gold, Heparin (green), EDTA (lavender), Gray
The Rationale - Why This Order?
1. Blood Cultures FIRST
- They require a sterile specimen - drawing them first, before the needle has contacted any tube stopper, minimizes contamination risk
- SPS in yellow-top culture tubes is anticoagulant + anti-complement; any carryover to other tubes would be a problem but doesn't happen since cultures go first
2. Light Blue (Sodium Citrate) SECOND
- Sodium citrate chelates calcium to prevent clotting - it is the most concentration-sensitive tube
- The ratio of blood to citrate must be exactly 9:1 - any contamination alters the ratio and falsifies coagulation results
- Discard tube rule: When using a butterfly/winged infusion set, draw a non-additive discard tube first to purge air from the tubing. If air enters the citrate tube, it reduces blood volume and creates a wrong blood:citrate ratio → falsely prolonged PT/aPTT
- It comes after blood cultures (not first) because puncturing culture bottle caps could introduce trace tissue thromboplastin into the line, which would contaminate coagulation tests
3. Serum Tubes (Red / Gold / SST) THIRD
- Allow blood to clot completely (30 min at room temp before centrifugation)
- Clot activators (silica or glass particles in SST) accelerate clotting
- Placed after citrate to avoid citrate carryover into serum, which would falsely lower ionized calcium and interfere with many chemistry assays
- Gel separator physically separates serum from cells after centrifugation
4. Green (Heparin) FOURTH
- Heparin inhibits thrombin and factor Xa to prevent clotting - yields plasma (not serum)
- Drawn after serum tubes because heparin carryover into a coagulation tube (light blue) would falsely prolong PT/aPTT
- Drawn before EDTA because heparin carryover into EDTA tubes has minimal clinical effect compared to the reverse
5. Lavender / Purple (EDTA) FIFTH
- EDTA chelates calcium AND magnesium with very high affinity
- EDTA is potassium-rich (K2EDTA or K3EDTA) - if an EDTA tube is drawn before a chemistry tube, EDTA carryover → falsely elevated potassium (pseudohyperkalemia)
- EDTA carryover also falsely lowers calcium, ionized calcium, iron, and can interfere with coagulation assays
- This is why EDTA always comes AFTER serum and heparin tubes
- EDTA does NOT interfere with blood bank testing (which uses EDTA), so pink EDTA comes here too
6. Grey (Fluoride/Oxalate) LAST
- Sodium fluoride inhibits glycolysis (preserves glucose)
- Potassium oxalate chelates calcium
- Both additives are highly disruptive - oxalate can falsely alter coagulation, potassium from oxalate can falsely elevate K+, and fluoride inhibits enzymes used in many assays
- Placing them last prevents this interference from contaminating any other tube
Special Situations & Pearls
| Situation | Rule |
|---|---|
| Butterfly set + citrate tube only | Draw a 3 mL discard tube first (eliminates dead-space air) |
| Drawing from an IV line | Flush with 5x dead-space volume of saline, discard 5 mL blood, then draw in standard order |
| Syringe draw | Fill tubes in the same order of draw when transferring; use a transfer device, not a needle |
| Trace metals / lead | Some labs (e.g., Mayo) draw these FIRST because needle back-end picks up trace metals from other stoppers |
| ACD (yellow) tubes | Go after EDTA - ACD contains citrate, dextrose, and citric acid for cell preservation |
| Capillary/fingerstick order | Different! EDTA goes FIRST (before clotting begins), then other tubes |
Why Additive Carryover Happens
When the needle withdraws from a filled tube and punctures the next tube stopper, a tiny droplet of the previous tube's contents can be pulled into the next tube. This is called "backflow" or carryover contamination. Modern tube design minimizes this with one-way valves, but the order of draw still matters as a safety net.
Capillary Blood Order of Draw (Fingerstick/Heelstick)
This is the REVERSE of venous order for some tubes because capillary blood clots rapidly:
- Blood gas (if needed)
- EDTA tubes (lavender) - FIRST in capillary
- Other additive tubes
- Serum tubes (no additive / red)
EDTA goes first in capillary collection because clotting begins immediately, and EDTA prevents this - ensuring an accurate CBC.
Quick Memory Aid
"Stop - Look - Right - Have - Everything - Good" Sterile (blood cultures) → Light blue (citrate) → Red/gold (serum) → Heparin (green) → EDTA (lavender/pink) → Gray (fluoride)
The overarching principle: draw tubes in order from least disruptive additive to most disruptive additive, with sterile specimens always first.
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