Phlebotomy is the process of drawing blood from a patient to obtain diagnostic specimens. To ensure the accuracy of laboratory tests, a specific collection method known as the “order of draw” must be followed. This standardized sequence dictates which collection tube must be filled before the next, ensuring the integrity of every specimen. Adhering to this precise order is necessary for accurate testing and proper patient care.
The Critical Role of Additive Carryover Prevention
The justification for the strict order of draw is the prevention of “additive carryover,” or cross-contamination. Blood collection tubes contain various chemical additives designed to preserve the sample or prevent clotting. If a tube containing an additive is collected before another, trace amounts can cling to the shared needle. This residue is then inadvertently introduced into the next sample when the needle pierces the stopper. Even minute quantities of a chemical can interfere with sensitive laboratory assays. The standard sequence arranges the tubes so that any unavoidable carryover involves an additive that will least affect the tests in the subsequent tube.
The Official Sequence: Tube Color, Additive, and Purpose
The standardized order of draw begins with sterile tubes or bottles designed for blood cultures, often with yellow tops. These are collected first to prevent contamination of the sample with bacteria from the skin or non-sterile tube stoppers. Blood cultures use Sodium Polyanethol Sulfonate (SPS) or a nutrient broth to isolate and identify microorganisms in cases of suspected bloodstream infections.
The second tube in the sequence is the light blue-top tube, used for coagulation studies such as the Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). This tube contains the anticoagulant Sodium Citrate, which works by binding to calcium, a substance necessary for clotting. Because the ratio of blood to additive is precisely controlled, this tube must be filled completely to maintain specimen integrity.
Following the light blue tube are the serum tubes, which include red, gold, or tiger-top (red-gray speckled) tubes. These tubes often contain a clot activator, such as silica particles, to rapidly initiate clotting. They may also include a gel barrier to separate the serum from the cells after centrifugation. Serum tubes are used for general chemistry panels, serology, and immunology studies.
Next are the green-top tubes, which contain the anticoagulant Heparin, usually as sodium or lithium heparin. Heparin prevents clotting by inhibiting thrombin, a key enzyme in the coagulation cascade. These tubes are used for routine and stat chemistry tests, allowing for rapid testing without the delay required for a serum tube to clot.
The fifth tube is the lavender or pink-top tube, which contains Ethylenediaminetetraacetic Acid (EDTA). EDTA is a strong anticoagulant that binds calcium and is used for hematology tests, such as the Complete Blood Count (CBC), because it preserves the cellular components of the blood. Pink-top tubes are used for blood bank compatibility testing.
The final tube in the sequence is the gray-top tube, which contains both Sodium Fluoride and Potassium Oxalate. Sodium Fluoride acts as an anti-glycolytic agent, stopping blood cells from consuming glucose, which is important for accurate blood sugar measurements. This tube is reserved for glucose, blood alcohol, and lactate testing, ensuring the preservation of the analyte until analysis.
Analyzing the Results of Procedural Errors
Violating the correct order of draw introduces specific errors that can lead to misdiagnosis or unnecessary treatment. A common error is drawing a lavender-top (EDTA) tube before a chemistry tube. Since EDTA is a salt of potassium, its carryover can cause a falsely elevated potassium level (factitious hyperkalemia). EDTA also binds calcium, leading to a factitiously low calcium result (hypocalcemia), which could prompt inappropriate medical intervention.
If a clot activator from a serum tube contaminates the light blue-top tube, coagulation study results will be inaccurate. The clot activator will prematurely shorten the clotting time, resulting in a falsely shortened PT or aPTT. Similarly, if blood culture bottles are not drawn first, non-sterile additives or bacteria can contaminate the culture media, leading to a false positive result and unnecessary antibiotic administration. Following the established order protects against these risks.