Blood collection, or venipuncture, is a routine medical procedure for diagnostic testing. Blood samples are collected into various tubes, each designed for specific tests and containing particular additives. The sequence in which these tubes are filled, known as the “order of draw,” is critical. This specific order prevents cross-contamination between tube additives, which could otherwise compromise sample integrity and lead to inaccurate laboratory results. Understanding this standardized procedure is important for reliable diagnostic outcomes.
The Science Behind the Sequence
The necessity for a specific order of draw stems from the diverse additives present in blood collection tubes, each designed for particular laboratory analyses. These additives, such as anticoagulants, clot activators, and gels, can interfere with tests if even minute amounts are carried over from one tube to another. Additive carryover occurs when the needle transfers some blood-additive mixture from one tube to the next. This contamination can significantly alter test results.
For instance, ethylenediaminetetraacetic acid (EDTA), a common anticoagulant found in lavender-topped tubes, works by binding to calcium, preventing blood from clotting. If EDTA is carried over into a tube intended for calcium level assessment, it can falsely decrease the measured calcium concentration. Similarly, heparin, another anticoagulant, functions by enhancing the activity of antithrombin III, which inhibits clotting factors. Heparin contamination can affect coagulation tests, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), leading to inaccurate clotting times. It can also interfere with enzyme assays and electrolyte measurements like potassium.
Conversely, tubes designed to promote clotting, often containing silica particles as a clot activator, are also a source of potential interference. Silica accelerates the clotting process necessary for obtaining serum. If these clot activators are introduced into tubes intended for coagulation studies, they can prematurely activate clotting factors, leading to falsely shortened coagulation times. The precise order of draw minimizes these risks by ensuring that additives with the highest potential for interference are drawn after those less susceptible to contamination or those where trace amounts would have minimal impact.
The Standardized Procedure
To counteract the risks of additive carryover, a universally accepted “order of draw” has been established by organizations like the Clinical and Laboratory Standards Institute (CLSI). This standardized sequence ensures optimal sample integrity for a wide range of laboratory tests. The general order typically begins with sterile blood culture tubes, which are collected first to prevent contamination by other tube additives or skin flora, which could lead to false positive results for bloodstream infections.
Following blood cultures, coagulation tubes, typically with a light blue stopper and containing sodium citrate, are drawn. Sodium citrate is an anticoagulant that preserves the blood’s clotting factors for accurate coagulation studies. Drawing these tubes early is important because other additives could interfere with coagulation results. Next in the sequence are serum tubes, which may or may not contain a clot activator and gel separator, usually indicated by red or gold stoppers. These tubes are designed to allow the blood to clot and separate the serum for chemistry and immunology tests.
Heparin tubes, identified by green stoppers, are typically drawn after serum tubes. These tubes contain heparin, an anticoagulant, and are used for plasma chemistry tests. Following heparin tubes are EDTA tubes, commonly recognized by lavender or pink stoppers. EDTA prevents clotting by binding to calcium and is primarily used for hematology tests, such as complete blood counts, where cellular integrity is crucial. Finally, tubes containing glycolytic inhibitors, such as sodium fluoride and potassium oxalate (gray stoppers), are drawn. These inhibitors prevent glucose metabolism in the sample, making them suitable for glucose testing.
Impact on Test Accuracy and Patient Care
Deviating from the correct order of draw can lead to inaccurate laboratory results, which have significant implications for patient care. When additives contaminate samples, test values can be falsely elevated or decreased, leading to misleading diagnostic information. For example, if EDTA contaminates a sample for potassium testing, it can cause a falsely elevated potassium level. Such inaccuracies can result in misdiagnosis, where a patient is diagnosed with a condition they do not have, or a delayed diagnosis, where a real condition is missed or its identification is postponed.
Misdiagnosis or delayed diagnosis can lead to inappropriate or unnecessary treatments, which may harm the patient or worsen their condition. For instance, a falsely low calcium level might prompt unnecessary calcium supplementation, while a falsely elevated potassium could lead to inappropriate medical interventions. Incorrect results often necessitate repeat blood draws, causing additional discomfort and inconvenience for the patient. These errors also increase healthcare costs due to duplicated tests and potentially prolonged hospital stays. The reliability of diagnostic testing directly influences medical decision-making, emphasizing why adherence to the order of draw is fundamental for patient safety and effective healthcare.