Recall the standard order of draw, why it prevents additive carry-over, and how to handle tubes after collection.
Why it matters
Blood collection tubes contain different additives: anticoagulants, clot activators, gels and preservatives. When several tubes are filled from one needle, a tiny amount of additive can be carried from one tube into the next. If EDTA from a lavender tube gets into a chemistry tube, it binds calcium and adds potassium, so the report may show a dangerously high potassium and a low calcium that the patient does not really have. The order of draw exists to prevent that carry-over.
Safety and scope. Venipuncture is a clinical skill learned in an accredited phlebotomy, MLT or MLS program and practiced only under supervision on consenting patients, after bloodborne-pathogen and sharps training. Never practice on yourself or anyone else at home. This lesson is knowledge only; nothing here is medical advice.
The core idea: the standard order
The order below follows the CLSI collection standard used by most US labs (always check your own lab's procedure, which is the version you follow):
| Order | Typical cap color | Additive | Common uses |
|---|---|---|---|
| 1 | Blood culture bottles | Culture medium | Blood cultures (drawn first to stay sterile) |
| 2 | Light blue | Sodium citrate | Coagulation (PT/INR, aPTT) |
| 3 | Red or gold | None, or clot activator with or without gel | Serum chemistry, serology |
| 4 | Green | Heparin (lithium or sodium) | Plasma chemistry |
| 5 | Lavender or pink | EDTA | CBC, HbA1c, blood bank |
| 6 | Grey | Sodium fluoride / potassium oxalate | Glucose, lactate |
Cap colors are conventions and can vary by manufacturer and country, so the label on the tube (the additive name) is what counts.
Why this order works
- Cultures first, because sterility matters most and nothing should touch the needle before them.
- Citrate early, because coagulation tests are sensitive to clot activators and to tissue factor. If a butterfly set is used and citrate is the first tube, a discard tube is drawn first to fill the tubing's dead space, so the citrate tube still fills to the correct 9:1 ratio.
- Serum tubes before plasma tubes, so clot activator is not carried into anticoagulated tubes.
- EDTA and fluoride/oxalate last, because their contamination does the most damage: EDTA (as potassium EDTA) raises potassium and binds calcium and magnesium; oxalate and fluoride also bind calcium and inhibit enzymes.
Handling after the draw
Order is only half the job. Each tube must also be:
- Filled correctly. Vacuum tubes are designed to draw the right volume for the additive.
- Inverted gently the number of times the manufacturer specifies (often 3 to 4 for citrate and 5 to 10 for others). Shaking causes hemolysis; not mixing lets anticoagulated tubes clot.
- Labelled at the patient's side immediately after collection (see the next lesson).
- Transported within the time and temperature limits for each test.
Worked example: planning a draw
A requisition (fictional) asks for blood cultures, PT/INR, a basic metabolic panel in a gold tube, a CBC and a glucose in a grey tube. Using a butterfly set, the plan is:
- Blood culture bottles (aerobic, then anaerobic, per the manufacturer).
- Light blue (citrate). A discard tube is not needed here, because the culture bottles already filled the tubing; if cultures were not ordered, a discard tube would be drawn first.
- Gold (serum, clot activator and gel).
- Lavender (EDTA).
- Grey (fluoride/oxalate).
If the lab later sees a potassium of 9.5 mmol/L with a calcium near zero in the gold tube (a combination incompatible with life in a patient who is talking), EDTA contamination is the first suspect. The lab would reject the result and request a redraw, not call it as a critical value.
A memory aid
Many students remember the sequence as "Stop, Light, Red, Stay Put, Green Light, Go": Sterile (cultures), Light blue, Red, SST (gold), Green, Lavender, Grey. Any mnemonic is fine as long as you can explain why each tube sits where it does; the reasons are what help you troubleshoot.
Common mistakes
- Drawing the EDTA tube before chemistry tubes, causing false high potassium and false low calcium.
- Forgetting the discard tube when citrate is first with a butterfly set, so the citrate tube is under-filled.
- Trusting cap color instead of reading the additive on the label.
- Shaking tubes vigorously instead of gentle inversion.
Quick recap
- Order of draw prevents additive carry-over: cultures, citrate, serum, heparin, EDTA, fluoride/oxalate.
- EDTA contamination looks like very high potassium with very low calcium.
- Fill, invert gently, label at the patient's side, and transport on time.
- Phlebotomy itself is learned and practiced only in supervised clinical training.
Try it
Open the Do tab for the quiz: sequence a draw and diagnose a contaminated result.