The Vacutainer system uses a pre-measured vacuum to draw blood samples directly into specialized tubes. This standardized method ensures the correct ratio of blood to the tube’s internal chemical additive. To prevent errors, each tube stopper is color-coded, indicating the specific chemical agent contained within.
The Function of Tube Additives and Color Coding
The color of a blood collection tube indicates the type of additive present, which determines how the sample will be processed and analyzed. Additives fall into two categories: anticoagulants, which prevent clotting, and clot activators, which accelerate the natural clotting process. Anticoagulants are used when whole blood or plasma (the liquid component containing clotting factors) is required for testing. Clot activators produce serum, which is the liquid portion remaining after cellular components and clotting factors have been removed through coagulation.
Anticoagulants interrupt the coagulation cascade through distinct chemical mechanisms. Ethylenediaminetetraacetic acid (EDTA), found in lavender tubes, prevents clotting by binding with calcium ions, a mineral required for clot formation. Heparin, found in green tubes, enhances the activity of antithrombin, a protein that inhibits clotting factors. Sodium citrate, used in light blue tubes, also binds calcium reversibly, allowing the clotting mechanism to be reactivated for specific laboratory testing. Tubes designed to produce serum often contain silica particles, which accelerate the clotting cascade.
Tube Colors and Their Corresponding Laboratory Tests
The light blue tube, containing sodium citrate, is used exclusively for coagulation studies, such as the Prothrombin Time (PT) and the Activated Partial Thromboplastin Time (aPTT). These tests measure how quickly blood clots. Because the ratio of blood to sodium citrate must be precisely 9:1, these tubes must be filled completely to ensure accurate results.
Serum Tubes
Red, gold, or tiger-top tubes are designed to yield serum for chemistry and serology tests. These tubes contain a clot activator to speed up the clotting process. Many also include a polymer gel barrier that, after centrifugation, separates the heavier blood cells from the lighter serum. The resulting serum is used for tests such as the Comprehensive Metabolic Panel, hormone level measurements, therapeutic drug monitoring, and blood banking procedures.
Lavender or Purple Tubes
The lavender or purple tube contains the anticoagulant EDTA, which preserves cellular integrity by binding calcium. This tube is the standard for hematology testing, where blood cells must remain intact for analysis. The most common test is the Complete Blood Count (CBC), which measures the number and characteristics of red cells, white cells, and platelets. The EDTA tube is also used for Hemoglobin A1c (HgbA1c) testing and blood typing.
Green Tubes
Green-top tubes contain heparin, typically as a lithium or sodium salt, and are used when plasma is required quickly for immediate (“Stat”) chemistry measurements. Heparin allows the blood to be centrifuged immediately to separate plasma for rapid analysis. This tube is used for tests like blood gas analysis, ammonia levels, and certain electrolyte panels. Some green tubes also include a gel separator to obtain a cleaner plasma sample.
Gray Tubes
The gray tube is specialized for glucose and lactate testing, containing a dual additive system: potassium oxalate and sodium fluoride. Potassium oxalate acts as an anticoagulant. Sodium fluoride functions as an anti-glycolytic agent, inhibiting enzymes that consume glucose after the sample is drawn. This prevents the glucose level from artificially decreasing over time, ensuring the result reflects the patient’s true blood sugar level.
The Order of Draw for Accurate Results
The sequence in which blood collection tubes are filled is known as the “Order of Draw.” This standardized procedure prevents the carryover of chemical additives between tubes. Contamination occurs because the needle, coated with the previous tube’s additive, is inserted into the next tube, which can compromise test accuracy.
The standardized sequence prioritizes tubes where contamination would be most damaging. For example, a tiny amount of EDTA, which binds calcium, would invalidate results if carried into a light blue tube intended for coagulation studies.
The typical order of draw is:
- Blood culture bottles
- Light blue coagulation tube
- Serum tubes (red, gold, or tiger-top)
- Heparin tubes (green)
- EDTA tubes (lavender)
- Gray tube for glucose testing