A blood draw, also known as phlebotomy or venipuncture, is a common medical procedure where a small blood sample is collected from a vein, typically in the arm. This process provides healthcare professionals with insights into the body’s functions, aiding in diagnosing conditions, monitoring diseases, and assessing treatment effectiveness.
Why the Order Matters
The “order of draw” is the specific sequence for collecting blood samples, crucial for accurate test results. Blood collection tubes contain various additives, such as anticoagulants, clot activators, and gels, each designed to preserve the sample or prevent clotting for specific laboratory analyses.
If blood is drawn incorrectly, “carryover” can occur, where a small amount of additive transfers from one tube to the next. Even minute amounts can interfere with subsequent samples, compromising results. Following a standardized order minimizes contamination and preserves specimen integrity.
The Standard Order of Draw
The process typically begins with sterile blood culture bottles, collected first to avoid contamination that could affect bacterial growth detection. These bottles are used to identify infections in the bloodstream.
Next in sequence are light blue-top tubes, which contain sodium citrate, an anticoagulant that prevents blood clotting by binding to calcium. These tubes are used for coagulation studies, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (APTT), which assess the blood’s clotting ability. It is important to fill these tubes completely to maintain the precise blood-to-anticoagulant ratio required for accurate results.
Following the light blue tubes are serum tubes, often identified by red, gold, or “tiger” (red-speckled) tops. Red-top tubes generally have no additive or contain a clot activator, while gold or tiger-top tubes contain a clot activator and a gel separator. These tubes allow the blood to clot, and after centrifugation, the serum is separated for various chemistry, immunology, and serology tests, including general chemistry panels and hormone assays.
Green-top tubes come next and contain heparin (sodium or lithium heparin) as an anticoagulant, which works by inhibiting thrombin to prevent clotting. These tubes are used for plasma chemistry tests, such as electrolyte measurements, liver function tests, and certain enzymatic assays. After the green tubes, lavender or pink-top tubes are typically drawn. These contain Ethylenediaminetetraacetic acid (EDTA), an anticoagulant that binds calcium to prevent clotting, preserving cell morphology. Lavender tubes are primarily used for hematology tests like complete blood counts (CBC) and hemoglobin A1c, while pink tubes are specific for blood banking and transfusion services.
Finally, gray-top tubes are collected, containing sodium fluoride and potassium oxalate. Sodium fluoride acts as a preservative by inhibiting glycolysis, which prevents glucose breakdown in the sample, while potassium oxalate serves as an anticoagulant. These tubes are primarily used for glucose testing, lactate levels, and blood alcohol measurements.
Potential Issues from Incorrect Order
Additive carryover, even in minute quantities, can cause false elevations or decreases in various measured values. For instance, if an EDTA-containing tube (lavender/pink) is drawn before a tube for potassium testing, the high potassium content in EDTA can falsely elevate the patient’s potassium level. Similarly, EDTA carryover can falsely decrease calcium and magnesium levels because EDTA binds to these ions.
Contamination from clot activators can lead to inaccurate coagulation test results, making it appear as though blood is clotting too quickly or too slowly. This can result in a misdiagnosis of bleeding or clotting disorders. Incorrect results may necessitate repeat blood draws, causing additional discomfort for the patient and delaying diagnosis and appropriate treatment. Accurate test results are fundamental for proper patient management, and errors due to an incorrect order of draw can have consequences such as unnecessary medical interventions or delayed life-saving treatments.