The color-coded stoppers on blood collection tubes are a standardized system designed to promote patient safety and ensure the accuracy of diagnostic testing. These tubes are intricate chemical environments tailored for specific laboratory analyses. The color of the cap instantly communicates the chemical contents within the tube to healthcare professionals, preventing errors and safeguarding the reliability of test results.
Why Tube Additives Require Standardization
Different medical tests require blood samples to be in different physical states for accurate analysis. Some tests require the blood to clot naturally, allowing the liquid serum portion to be separated, while others require the blood to remain in its whole, liquid form to count cellular components. To achieve these varied states, specialized chemical additives are coated or contained inside the tubes. These additives are primarily categorized as either anticoagulants, which stop blood from clotting, or clot activators, which speed up the clotting process.
Anticoagulants, such as Ethylenediaminetetraacetic acid (EDTA) or Heparin, interfere with the body’s complex clotting cascade. Clot activators, often silica particles, provide a surface that encourages rapid clotting. The color-coding system provides a clear, universal identifier for the specific chemical present, dictating the type of test the sample is valid for. This system allows technicians to quickly match the required sample state to the appropriate tube, ensuring specimen integrity.
Decoding the Most Common Stopper Colors
The most common stopper colors encountered in routine blood draws each correspond to a specific additive and test category.
Lavender/Purple
The Lavender or Purple top tube contains EDTA, a powerful anticoagulant that prevents clotting by binding to calcium ions necessary for coagulation. Because EDTA preserves the morphology and structure of blood cells, this tube is exclusively used for hematology tests, such as a Complete Blood Count (CBC).
Light Blue
The Light Blue top tube contains Sodium Citrate, a milder anticoagulant used specifically for coagulation studies, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). Sodium citrate works by binding calcium, but the concentration is calibrated to be reversible in the lab, allowing the clotting process to be measured. It is important that this tube is filled precisely to the indicated line, as an incorrect blood-to-additive ratio will skew results significantly.
Red/Gold/Tiger Top
Tubes with Red or Gold/Tiger Top stoppers are used when serum is needed, which is the fluid portion of blood remaining after clotting. The plain Red top tube typically contains no additive, allowing the blood to clot naturally before centrifugation. The Gold or Tiger Top tubes, often referred to as Serum Separator Tubes (SST), contain a clot activator and a separation gel. Once centrifuged, the gel forms a physical barrier between the clotted blood cells and the lighter serum, making it ideal for routine chemistry and serology tests.
Green
The Green top tube contains Heparin, an anticoagulant that prevents clotting by enhancing the activity of antithrombin III. This tube is used for chemistry tests that require plasma rather than serum, especially when results are needed quickly. The rapid action of Heparin makes it suitable for tests like ionized calcium or certain electrolyte panels.
Ensuring Sample Integrity The Order of Draw
The color-coding system is also fundamental to maintaining sample integrity through a procedural guideline known as the “Order of Draw.” This sequence dictates the precise order in which tubes must be filled during a blood collection procedure. The order is standardized to prevent trace amounts of additive from one tube from contaminating the blood sample in the next tube.
For example, EDTA could interfere with calcium-dependent coagulation tests if it contaminated the light blue tube. Following this sequence, established by organizations like the Clinical and Laboratory Standards Institute (CLSI), minimizes the risk of cross-contamination from residual additive on the needle’s tip. Adherence to the Order of Draw is necessary to ensure that laboratory results are accurate and reliable, directly impacting patient diagnosis and care.