The color-coded system of blood collection tubes, often called Vacutainers, is a standardized, globally used method. This system ensures blood samples are collected and prepared correctly for laboratory analysis. Since different medical tests require the blood to be in a specific state—clotted, unclotted, or separated—the tube stopper’s color signals the exact chemical additive inside. This allows healthcare professionals to quickly select the correct container, which is fundamental to ensuring accurate diagnostic results.
The Role of Additives in Blood Preservation
Blood collection tubes contain specific additives that determine the final state of the blood sample for testing. The fundamental difference is whether the tube allows the blood to clot or prevents clotting. Tubes that allow clotting, often containing a clot activator, produce serum after centrifugation separates the heavier cellular components. Serum is the clear, liquid part of the blood remaining after clotting factors have been consumed.
Tubes that prevent clotting contain an anticoagulant, keeping the blood in its liquid state. When centrifuged, these samples yield plasma, the liquid portion of the blood that still contains clotting factors. Anticoagulants interfere with the body’s natural coagulation cascade, often by binding to calcium ions or inhibiting specific clotting proteins. This distinction is crucial because certain tests require whole blood or plasma to analyze cellular components or clotting factors, while others require serum for chemistry and immunologic assays.
Decoding the Primary Tube Colors and Associated Tests
The color of the tube stopper directly corresponds to the additive, the resulting sample type, and the categories of tests that can be performed. This visual cue is the primary instruction for proper sample collection.
Red/Gold/Tiger Top (SST)
These tubes are designed to yield serum for a wide range of chemistry and serology tests. The red-top tube may contain no additive, allowing the blood to clot naturally, or it may contain a clot activator. The gold or “tiger top” tubes contain both a clot activator (often silica particles) and a separation gel (SST). The clot activator accelerates clotting, and the gel forms a stable barrier between blood cells and the clear serum after centrifugation. These samples are used for general chemistry panels, hormone assays, and therapeutic drug level monitoring.
Light Blue
The light blue-top tube contains the anticoagulant sodium citrate, which binds to calcium ions necessary for clotting. This tube is exclusively used for coagulation studies, which assess the blood’s ability to clot. Common tests include Prothrombin Time (PT), Activated Partial Thromboplastin Time (aPTT), and D-dimer. The ratio of blood to the sodium citrate additive is extremely precise (typically 9:1), making the fill level critically important. If the tube is underfilled, the excess anticoagulant falsely prolongs clotting times, leading to inaccurate results.
Lavender/Purple
Tubes with a lavender or purple stopper contain the anticoagulant ethylenediaminetetraacetic acid (EDTA). EDTA prevents clotting by chelating (tightly binding) calcium ions within the blood. These tubes are used to obtain whole blood for hematology tests, where preserving cellular components is necessary. Common tests include the Complete Blood Count (CBC), blood typing, and Hemoglobin A1c.
Green
Green-top tubes contain the anticoagulant heparin, typically as a lithium or sodium salt. Heparin enhances the activity of antithrombin, a natural inhibitor of thrombin and other clotting factors. The resulting sample is plasma, often used for rapid (STAT) chemistry tests. Since plasma can be separated and tested immediately without waiting for clotting, green tubes are useful for emergency situations requiring quick results for electrolytes, cardiac markers, or basic metabolic panels.
Gray
The gray-top tube commonly contains two additives: potassium oxalate and sodium fluoride. Potassium oxalate acts as an anticoagulant, while sodium fluoride is a glycolytic inhibitor. Sodium fluoride prevents blood cells from consuming glucose in the sample (glycolysis), which stabilizes the glucose level. This tube is primarily used for glucose testing, such as Fasting Blood Sugar or Glucose Tolerance Tests, especially when sample transport to the laboratory is delayed.
Ensuring Accuracy: The Critical Order of Draw
The sequence in which blood collection tubes are filled is known as the “Order of Draw.” This standardized procedure is designed to prevent additive cross-contamination between tubes. Since the needle used to puncture one stopper may retain minute amounts of additive, this residue could be transferred into the next tube drawn. This carryover, even in trace amounts, can significantly compromise the accuracy of test results.
For example, EDTA from a lavender tube can interfere with calcium-dependent clotting factors in a light blue tube, leading to falsely prolonged coagulation times. Similarly, the silica clot activator from a serum tube could be carried over into a coagulation tube, causing microclots that skew results. The standard sequence is arranged to minimize the impact of the most interfering additives.
The order typically begins with sterile samples (e.g., blood cultures), followed by the light blue (coagulation) tube. Next are serum tubes (red, gold, or tiger top), followed by heparin (green), EDTA (lavender/purple), and finally the glycolytic inhibitor (gray) tube. Adhering to this established order ensures that sensitive coagulation tests are not contaminated by clot activators and that chemistry tests are protected from the interfering effects of EDTA and sodium fluoride.