Vacuum tubes used for blood collection are standardized globally and designed with specific chemical environments inside. The color of the tube cap serves as an immediate signal to phlebotomists and lab technicians regarding the tube’s contents and intended use. This system ensures that the blood sample is correctly preserved or processed to yield reliable data for diagnosis and monitoring.
The System: Color, Additive, and Sample Type
The color-coding system dictates the type of additive present within the tube, which determines the final sample type required for testing. Additives fall into two broad categories: clot activators (which promote clotting) and anticoagulants (which prevent it). Blood collected into a tube without an anticoagulant will naturally clot, and spinning this sample down yields serum, the fluid portion lacking clotting factors. Conversely, tubes containing an anticoagulant prevent clotting, and spinning this blood produces plasma, a fluid that retains all clotting factors. When the blood is not spun down, the result is whole blood, analyzed with all cellular components intact.
Routine Tubes for Common Screening
The lavender or purple-top tube is used for measuring the cellular components of blood. This tube contains the anticoagulant Ethylenediaminetetraacetic acid (EDTA), which functions by tightly binding to calcium ions, halting the entire coagulation cascade. Since EDTA preserves the size and shape of red and white blood cells most effectively, the resulting whole blood sample is used primarily for a Complete Blood Count (CBC) and for blood typing procedures.
The light blue-top tube is reserved for evaluating the body’s ability to stop bleeding, requiring the anticoagulant sodium citrate. Citrate works by reversibly binding calcium, which is reintroduced later in the lab to initiate the clotting reaction under controlled conditions. This mechanism is necessary for accurate coagulation studies, such as the Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). The test requires a precise ratio of nine parts blood to one part anticoagulant, yielding plasma before analysis.
Routine chemistry and serology tests, including the Comprehensive Metabolic Panel (CMP) and antibody screenings, use the gold, tiger, or sometimes plain red-top tubes. These tubes contain a clot activator to speed up the natural clotting process. The gold or tiger variants include a gel barrier to physically separate the clot from the resulting serum after centrifugation. The red-top tube, which usually contains only a clot activator and no gel, is utilized for collecting serum for specialized tests.
Specialized Tubes for Unique Diagnostics
The green-top tube, containing heparin (usually sodium or lithium heparin), is employed when a rapid turnaround time is needed for chemistry tests. Heparin acts by enhancing the activity of antithrombin, neutralizing clotting factors quickly. This allows the sample to be immediately spun down, yielding plasma for urgent (STAT) testing of analytes like troponin or basic electrolytes. It is generally avoided for routine chemistry due to potential interference with certain enzyme assays.
For accurate glucose measurements, the gray-top tube is mandated because it contains a dual additive: potassium oxalate (anticoagulant) and sodium fluoride (glycolysis inhibitor). Blood cells continue to consume glucose after the blood is drawn (glycolysis), artificially lowering the measured sugar level. Sodium fluoride stops this metabolic process, ensuring the reported glucose value is accurate. This tube is also the standard for measuring blood alcohol levels.
Yellow-top tubes, often containing Acid Citrate Dextrose (ACD) or Sodium Polyanethol Sulfonate (SPS), are specialized for maintaining the viability of microorganisms or the integrity of cellular components. The SPS additive, used in blood culture bottles, helps prevent immune cells from destroying bacteria, allowing for successful growth and identification. The ACD solution preserves red blood cells, making it the preferred medium for specialized cell studies, such as paternity testing and tissue typing.
The Critical Role of Drawing Order
The sequence in which the vacuum tubes are filled, known as the “Order of Draw,” is a precise procedural mandate designed to prevent the cross-contamination of additives between tubes. If a small amount of an additive is carried over, it can severely compromise the accuracy of the test results. For instance, the high concentration of potassium in EDTA (purple top) can contaminate chemistry tests, or its calcium-binding action can invalidate coagulation results (light blue top).
The standard sequence generally begins with blood culture bottles or yellow SPS tubes, followed by the light blue sodium citrate tube. Next are the serum tubes (red or gold), then the green heparin tubes, and finally the purple EDTA tubes. The gray sodium fluoride tube is typically last.