Venipuncture is the procedure of obtaining a blood specimen from a vein, commonly referred to as a blood draw. This collection process is a fundamental step in medical diagnosis, providing samples for a vast array of laboratory tests. The reliability of these results depends heavily on following a precise protocol known as the “order of draw.” This standardized sequence dictates the exact order in which different blood collection tubes must be filled during a single procedure to prevent chemical interference that could lead to erroneous laboratory findings.
Why Tube Contamination Must Be Avoided
The strict order of draw exists to prevent the chemical additives contained in one collection tube from being carried over into the next tube. Every blood collection tube is manufactured with specific substances designed to prepare the blood sample for a particular type of analysis. These substances are not interchangeable and are formulated to either prevent the blood from clotting or to accelerate the clotting process.
The phenomenon of “carryover” contamination occurs when a minute amount of an additive coats the venipuncture needle as the first tube is removed, and that residue is then transferred into the subsequent tube. For example, the anticoagulant Ethylenediaminetetraacetic acid (EDTA), found in lavender-top tubes, contains high levels of potassium. If EDTA contaminates a sample intended for electrolyte testing, it can falsely elevate the patient’s potassium result, potentially leading to an incorrect diagnosis or unnecessary treatment.
Similarly, the clot activators present in serum tubes can be transferred to a tube meant for coagulation studies, which must remain in a liquid state. This contamination could cause micro-clots to form, leading to inaccurate coagulation times. Following the proper order ensures that any potential carryover of additives will not significantly interfere with the tests performed on the next tube in the sequence, thereby preserving the integrity of every specimen.
The Standard Venipuncture Sequence
The standard venipuncture sequence is established by organizations like the Clinical and Laboratory Standards Institute (CLSI) to minimize the risk of additive carryover. The order is based on placing tubes with the highest potential for interference later in the draw.
The first tubes collected are Blood Culture bottles, which must be drawn first to ensure maximum sterility. These samples are used to detect the presence of microorganisms in the blood, so any potential contamination from the skin or other tubes must be avoided.
The next tube is the Light Blue Top, which contains sodium citrate and is used for coagulation testing. This tube is highly sensitive to fill volume, as the ratio of blood to the liquid anticoagulant must be precise. When a butterfly needle is used, a non-additive discard tube must be drawn immediately before the light blue top to clear the air space from the tubing. This ensures the collection tube is filled entirely with blood, maintaining the correct anticoagulant-to-blood ratio for accurate results.
Following the coagulation tube are the tubes intended for serum collection, typically with Red, Gold, or Tiger Tops. These tubes contain clot activators and sometimes a gel separator, and they are placed here because their additives have minimal impact on subsequent tubes. Next come the Heparin tubes, usually with a Green Top, which contain the anticoagulant heparin.
Finally, the tubes containing the strongest anticoagulants are drawn last to prevent them from contaminating other samples. This includes the Lavender or Pink Top tubes, which contain EDTA and are used for hematology studies. The last tube in the standard draw is the Gray Top tube, which contains sodium fluoride and potassium oxalate, primarily for glucose and alcohol testing.
Common Tube Types and Their Purpose
Blood collection tubes are identified by the color of their stopper, which indicates the specific additive inside and the intended laboratory test.
- Light Blue Top: Contains sodium citrate. Used for coagulation testing (e.g., PT and PTT), requiring a precise blood-to-anticoagulant ratio.
- Serum Tubes (Red, Gold, Tiger Top): Contain a clot activator (often silica) but no anticoagulant. Used for chemistry and immunology tests after the blood has clotted and been centrifuged to separate serum.
- Green Top: Contains the anticoagulant Heparin. Used for general chemistry tests, yielding plasma samples.
- Lavender or Pink Top: Contains EDTA, a powerful chelating agent. Used for hematology studies, such as the Complete Blood Count (CBC), to preserve blood cells in a non-clotted state.
- Gray Top: Contains potassium oxalate (anticoagulant) and sodium fluoride (antiglycolytic agent). Primarily used for blood glucose and alcohol measurements.