The Clinical Rationale for Blood Collection Sequence
The Order of Draw is the standardized sequence in which blood collection tubes are filled during venipuncture. This sequence is critical to prevent cross-contamination of tube additives, which can alter test results. For medical assistants, mastering the order of draw is a core competency and a frequent exam topic on certification tests such as the CMA (AAMA) and RMA.[1]
Phlebotomy Terminology and Additive Roles
- Additive: A substance inside a blood collection tube that preserves, clots, or stabilizes blood for specific laboratory tests.
- Evacuated tube: A vacuum-sealed tube used to draw a precise volume of blood.
- Cross-contamination: The transfer of additives from one tube to another, which can occur if tubes are filled in the wrong order.
- Anticoagulant: An additive that prevents blood from clotting (e.g., EDTA, heparin, citrate).
- Serum: Blood without clotting factors—obtained after clotting and centrifugation of a tube without anticoagulant.
- Plasma: Liquid portion of blood obtained from a tube containing an anticoagulant.
Venipuncture Tube Sequence and Additive Rationale
Standard Order of Draw (CLSI GP41)[2]
- Blood culture tubes (sterile, yellow or clear top) – Collect first to avoid contamination.
- Light blue top – Contains sodium citrate; used for coagulation studies (PT, PTT).
- Red top (plain) or Gold top (serum separator gel) – No anticoagulant; used for serum chemistry, serology, and blood bank testing.
- Green top – Contains heparin (lithium, sodium, or ammonium); used for plasma chemistry and stat tests.
- Lavender top – Contains EDTA; used for hematology (CBC, ESR) and blood smears.
- Gray top – Contains potassium oxalate and sodium fluoride; used for glucose and lactate testing.
Rationale for the Sequence
- Blood cultures first to prevent microbial contamination from skin or environment.
- Light blue before tubes with other additives to avoid contamination with anticoagulants that could falsely alter coagulation results.
- Red/gold after coagulation tubes because they lack anticoagulants; carryover of citrate is minimal.
- Green after serum tubes to prevent heparin contamination of chemistry tests.
- Lavender after heparin tubes because EDTA can bind calcium and interfere with heparin assays if carried over.
- Gray last because fluoride is a potent antimetabolite and can affect other tests if carried over.
Tube Types, Additives, and Common Tests
| Tube Top Color | Additive | Common Tests | Inversions (gentle) |
|---|---|---|---|
| Yellow (blood culture) | Sterile (no additive) | Microbiology, blood cultures | None required |
| Light Blue | Sodium citrate (3.2%) | PT, aPTT, D-dimer | 3–4 |
| Red (plain) | None (glass) | Serum chemistry, blood bank, serology | 0 (allow to clot) |
| Gold (SST) | Silica clot activator + gel | Serum chemistry, liver panel, lipids | 5 |
| Green | Heparin (lithium/sodium) | Plasma chemistry, ammonia, stat electrolytes | 8–10 |
| Lavender | EDTA (K₂, K₃) | CBC, ESR, hemoglobin A1c, blood smears | 8–10 |
| Gray | Potassium oxalate + sodium fluoride | Glucose, lactate, ethanol level | 8–10 |
Note: Royal blue tops (for trace metals) are drawn before red tops if used. They may have either EDTA or heparin—follow facility protocol.[3]
Preventing Cross-Contamination and Hemolysis in Phlebotomy
- Cross-contamination: Filling tubes out of order can introduce additives into the next tube, altering test results (e.g., citrate in a lavender tube can falsely lower CBC).[4]
- Hemolysis: Excessive agitation, small-gauge needles, or prolonged tourniquet use can hemolyze samples, requiring redraws.
- Tube mixing: Invert each tube gently as specified above to ensure proper mixing of additive with blood. Failure to mix can cause clots or inaccurate results.
- Blood volume: Underfilled light blue tubes will have an altered blood-to-citrate ratio, invalidating coagulation results.
- Needle safety: Always activate safety features immediately after venipuncture and dispose of sharps in a designated container.[5]
Exam-Focused Mnemonics and Key Tube Protocols
- Memory Aids: "Blue before Red, Red before Lavender", or "Boys (Blood cultures) Love (Light blue) Ravishing (Red) Girls (Green) Like (Lavender) Grapes (Gray)".
- Know the inversions: Light blue (3–4), red/gold (5), green (8–10), lavender (8–10), gray (8–10).
- Remember that plain red top (no additive) does not need inversion; it must be allowed to clot upright.
- If a winged blood collection set is used, draw a discard tube (plain red) first to avoid air in the tubing, then follow the normal order.
- Blood culture tubes are always first—sterile technique is paramount.
- For coagulation studies (light blue), a complete fill is required; underfill is the most common error.
- Additive interference: Carryover of potassium from the gray tube can falsely elevate potassium in a subsequent tube if drawn out of order.
- On the exam, you may be asked to identify the correct order for a patient needing a CBC (lavender), PT (light blue), and basic metabolic panel (gold). The correct order: Blood culture → light blue → gold → lavender.
References & Sources
- Clinical and Laboratory Standards Institute (CLSI). GP41: Collection of Diagnostic Venous Blood Specimens. 7th ed. CLSI; 2017. https://clsi.org/standards/products/gp41/
- National Phlebotomy Association. Order of Draw. NPA; 2020. https://www.nationalphlebotomy.com/order-of-draw
- Garza D, Becan-McBride K. Phlebotomy Essentials for Medical Assistants. 6th ed. Pearson; 2018. https://www.pearson.com/en-us/subject-catalog/p/phlebotomy-essentials-for-medical-assistants/P200000009656
- Occupational Safety and Health Administration (OSHA). Bloodborne Pathogens Standard (29 CFR 1910.1030). US Department of Labor; 2021. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1030
- American Medical Technologists (AMT). RMA Certification Exam Content Outline. AMT; 2022. https://www.americanmedtech.org/Portals/0/PDF/2022/2022-RMA-Content-Outline.pdf