Blood tests are a common diagnostic tool, providing insights into health. Blood is collected into various tubes, each distinguished by a specific color on its stopper. These colors indicate the precise contents within, designed to preserve the blood sample for specific laboratory analyses. This system ensures samples are correctly prepared, contributing to accurate test results.
The Science Behind Tube Colors
Tube stopper colors are necessary because blood samples require varied treatments for stability. The color signifies additives inside the tube. These additives interact with blood to prevent clotting, promote clotting, or separate components. Their selection ensures sample integrity for precise measurements.
Anticoagulants prevent blood from coagulating. Clot activators encourage rapid clotting for tests requiring serum. Gel separators form a barrier between cellular components and plasma or serum after centrifugation, ensuring clean separation. Each additive prepares the sample for specific analytical methods.
Common Tube Colors and Their Associated Tests
Red Top Tube
Red top tubes contain no additives, allowing blood to clot naturally. They are used to obtain serum, the liquid portion of blood after coagulation. Tests include chemistry panels, therapeutic drug monitoring, and some serology. These tests rely on the absence of anticoagulants.
Gold Top Tube / SST (Serum Separator Tube)
Gold top tubes (SSTs) contain a clot activator and a gel separator. The clot activator accelerates clotting, and the gel forms a barrier between serum and blood cells after centrifugation. They are used for routine chemistry, immunology, and serology, providing a clean serum sample. The gel barrier maintains analyte stability by preventing interaction with cellular components.
Light Blue Top Tube
Light blue top tubes contain sodium citrate, an anticoagulant that prevents blood coagulation by binding to calcium ions. They are designed for coagulation studies like Prothrombin Time (PT), Activated Partial Thromboplastin Time (aPTT), and D-dimer tests. Accurate results depend on a precise blood-to-anticoagulant ratio, making proper fill volume important. Underfilling or overfilling can lead to inaccurate coagulation results.
Lavender / Purple Top Tube
Lavender or purple top tubes contain Ethylenediaminetetraacetic acid (EDTA), an anticoagulant that prevents clotting by chelating calcium. EDTA preserves blood’s cellular components, making these tubes ideal for hematology tests. Common tests include Complete Blood Count (CBC), Hemoglobin A1c (HbA1c), and Erythrocyte Sedimentation Rate (ESR). The integrity of blood cell morphology is maintained within these tubes.
Green Top Tube
Green top tubes contain heparin (sodium, lithium, or ammonium). Heparin acts as an anticoagulant by inhibiting thrombin, a protein in the clotting cascade. They are used for plasma chemistry tests, including STAT chemistry panels and ammonia levels. Since heparin allows for plasma collection, results can often be obtained more quickly than with serum samples.
Gray Top Tube
Gray top tubes contain sodium fluoride and potassium oxalate. Sodium fluoride acts as an antiglycolytic agent, inhibiting glucose metabolism by blood cells and preserving glucose concentration. Potassium oxalate serves as an anticoagulant. They are used for glucose and lactate measurements, where preventing glucose degradation is important for accurate results.
Royal Blue Top Tube
Royal blue top tubes are trace element-free, with extremely low levels of contaminating metals. These tubes may or may not contain an anticoagulant, depending on the specific test. They are designated for trace metal analysis (e.g., lead, copper, zinc), where minute contamination could skew results.
Importance of Proper Collection and Handling
Knowing the correct tube color is one aspect of accurate blood results; proper collection and handling are equally significant. Phlebotomists follow a specific “order of draw” to prevent cross-contamination between tube additives, which could compromise test integrity. This sequence ensures that the most sensitive tests are not affected by residues from other tubes.
After collection, tubes with additives must be gently inverted to ensure proper mixing. Vigorous shaking can cause hemolysis (red blood cell rupture), altering test results. Timely transport to the laboratory under appropriate temperature conditions is also necessary to maintain sample stability. Errors in these steps can lead to unreliable results, potentially requiring a repeat blood draw.
Beyond the Colors: Understanding Your Test Results
Blood collection, guided by the color-coded system, is the initial phase in a diagnostic process. Once samples arrive at the laboratory, professionals analyze them using specialized equipment. The laboratory then generates reports, transmitted to the healthcare provider.
Interpreting test results requires medical expertise and understanding a patient’s health context. Healthcare providers consider results with symptoms, medical history, and other diagnostic information to form a complete picture. Careful blood collection, facilitated by color-coded tubes, supports the accuracy of laboratory analyses, aiding informed medical decisions and patient care.