Order of draw is the single most important procedural concept in phlebotomy, and one of the most commonly mixed up. A wrong order can cause anticoagulant carryover that produces a false lab result and patient harm. This guide gives you the standard order, the rationale behind it, the best mnemonics to remember it, and the nuances that show up on ASCP PBT questions.
The Standard Order of Draw
The CLSI-recommended order for evacuated tube collection (ETS) is:
1. Blood cultures (yellow SPS or aerobic/anaerobic bottles)
2. Sodium citrate tube (light blue)
3. Serum tubes (red, gold/SST, tiger/tiger-top)
4. Heparin tubes (green, green/gray PST)
5. EDTA tubes (lavender/purple, pink)
6. Glycolytic inhibitor tubes (gray, yellow-gray)
A simplified order when blood cultures aren’t ordered: Light blue → Red/Gold → Green → Lavender → Gray
The Best Mnemonics for Order of Draw
Mnemonic 1: “Stop, Look, Right, Here, Get Going”
Map each word to a color:
Stop = Sodium citrate (light blue)
Look = Lavender… wait, this doesn’t quite work chronologically. Let’s use a better one:
Mnemonic 2: “Boy, Scout, Ready, Get, Lavender, Gray”
Blood cultures
Sodium citrate (light blue)
Red/Gold (serum)
Green (heparin)
Lavender (EDTA)
Gray (fluoride)
Mnemonic 3: The Color Story
Many phlebotomists prefer a vivid story: “The blue sky turned red, then green, then purple, then gray.”
Light blue → red (serum) → green (heparin) → lavender/purple (EDTA) → gray (fluoride)
Add blood cultures before this (“before the blue sky”) when needed.
Mnemonic 4: “Stop Being Royal, Great Ladies Get Going”
This one uses the first letter of each word to map to tube colors:
Stop, Sterile / blood cultures
Being, Blue (sodium citrate, light blue)
Royal, Red/serum
Great, Green (heparin)
Ladies, Lavender (EDTA)
Get Going, Gray (glycolytic inhibitor)
Choose whichever sentence sticks for you. The key is to tie the verbal cue to the actual tube color until it becomes automatic.
Why This Order Matters (The Rationale)
Mnemonics are only half the battle. The ASCP PBT exam asks scenario questions that require you to understand why the order exists, not just recite it. Here’s the rationale for each position:
Blood Cultures First
Blood culture collection requires sterile technique to detect true bacteremia. By collecting cultures first, you minimize skin flora contamination before any tube additives are introduced. A contaminated blood culture means a costly false-positive result, unnecessary antibiotics, and days of patient anxiety.
Sodium Citrate (Light Blue) Second
Coagulation tests require a precise 9:1 blood-to-citrate ratio. If EDTA or heparin contaminates the citrate tube, coagulation results are falsely prolonged. Sodium citrate must follow blood cultures (which have no carry-over risk) and precede all other anticoagulant tubes.
Important nuance: If the sodium citrate tube is the first tube drawn (no blood cultures ordered), CLSI recommends a 1-2 mL discard tube to clear the needle of tissue thromboplastin that entered during venipuncture. Some institutions skip this step; know your facility protocol.
Serum Tubes (Red/Gold) Third
Serum tubes come after the citrate tube because anticoagulant carryover into a serum tube is not clinically significant in either direction. The tube contains a clot activator (and gel separator in SST types) and does not have strict ratio requirements like the citrate tube.
Heparin (Green) Fourth
Heparin inhibits thrombin. If it contaminates an EDTA tube, it can cause errors in CBC testing. Placing heparin before EDTA prevents this carryover.
EDTA (Lavender) Fifth
EDTA chelates calcium to prevent clotting. EDTA carryover into heparin or citrate tubes would affect calcium levels and coagulation results, so EDTA comes late in the order. EDTA tubes require 8-10 gentle inversions immediately after collection.
Glycolytic Inhibitor (Gray) Last
Gray tops contain sodium fluoride and potassium oxalate. Fluoride inhibits glycolysis. These tubes are collected last because fluoride has the potential to affect certain enzyme tests if it contaminated earlier tubes. They also require 8 inversions.
Order of Draw for Capillary Collection
Capillary (fingerstick or heelstick) order of draw differs from venipuncture:
1. EDTA tubes first (to ensure accurate CBC before platelets clump)
2. Other additive tubes
3. Serum/clot tubes last
This is opposite from the venipuncture order for EDTA. The reason: capillary specimens contain tissue fluid, which can activate clotting. Collecting EDTA first preserves cellular morphology before platelet clumping compromises the CBC.
Exam Questions That Trip People Up
Q: When is a discard tube needed? When sodium citrate is the first tube collected in ETS (no blood cultures ordered). Draw and discard 1-2 mL before the citrate tube to clear tissue thromboplastin from the needle.
Q: Does order matter for syringe transfers? Yes. When transferring blood from a syringe to tubes, use the same order. Remove the needle, attach a blood transfer device, and fill tubes in the standard sequence.
Q: What if you draw EDTA before sodium citrate? EDTA carryover chelates calcium in the citrate tube, falsely prolonging PT/PTT. Recollect the coagulation specimen.
The Fastest Way to Lock In Order of Draw
Mnemonics help, but the fastest path to reliable recall is seeing order-of-draw questions over and over until the answer becomes automatic. Our free quiz includes order-of-draw questions with full rationale explanations. The full exam simulator has 150+ questions across all five ASCP PBT domains, with performance analytics showing your accuracy on equipment-domain questions specifically. Most students who struggle with order of draw questions find they need 20-30 repetitions to get to consistent accuracy, the simulator gets you there without wasted time.