Phlebotomy, commonly known as drawing blood, is a fundamental procedure in modern healthcare, providing samples necessary for diagnostic testing. Blood is collected into specialized tubes, each containing pre-measured chemical additives designed to preserve the sample or prepare it for a specific assay. These additives prevent clotting or inhibit metabolic processes until the sample reaches the laboratory. The Order of Draw (OOD) is a strict procedural safeguard that dictates the precise sequence in which these tubes must be filled during a venipuncture.
The Science of Additive Carryover
The strict sequence of the Order of Draw is necessary because of the risk of additive carryover, where a minute amount of chemical from one tube contaminates the next tube drawn. This contamination occurs at the point of needle exchange within the tube holder assembly used for vacuum collection. As the first tube is removed, residual material, such as liquid anticoagulant or clot activator powder, can remain on the needle tip that is temporarily exposed within the holder. When the next tube is inserted, this residue is flushed into the new blood sample, introducing an unwanted chemical that can interfere with the intended test. Even microscopic amounts of contamination, sometimes measurable in just a few microliters, can be sufficient to bias sensitive laboratory results.
Consequences of Incorrect Drawing Sequence
Failing to follow the correct drawing sequence leads to inaccurate test results, potentially resulting in misdiagnosis or incorrect patient treatment. The most common error involves the carryover of Ethylenediaminetetraacetic acid (EDTA) from lavender- or purple-topped tubes. EDTA is a powerful chelating agent that tightly binds to metal ions. If EDTA contaminates a chemistry sample, it binds free calcium and magnesium ions, resulting in a falsely low reading for calcium and magnesium (pseudohypocalcemia). Furthermore, EDTA carryover contains potassium salt, which causes a falsely elevated potassium result (spurious hyperkalemia) that can lead to harmful medical interventions.
Another significant issue arises from the clot activator, typically silica, present in serum tubes (red, gold, or tiger tops). If the light blue-topped tube, used for coagulation studies like the Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), is drawn after a serum tube, the clot activator can carry over. Silica is designed to speed up clotting, and its carryover falsely shortens the clotting time measurements. Similarly, Heparin, an anticoagulant from green-topped tubes, can interfere with coagulation studies if drawn out of sequence. Finally, drawing any non-sterile tube before the blood culture bottle risks contaminating the culture with bacteria from the preceding tube’s stopper.
The Standardized Order of Draw
To mitigate the risks of additive carryover, a standardized Order of Draw has been established, based on guidelines from organizations like the Clinical and Laboratory Standards Institute (CLSI). This sequence ensures that additives with the least potential for interference are drawn first, and tubes with the most problematic additives, like EDTA, are drawn last. The standardized order remains the same regardless of whether a vacuum system or a syringe is used for the collection.
Standardized Tube Sequence
The standardized sequence ensures the integrity of the blood samples:
- Sterile blood culture tube (must be drawn first to prevent contamination).
- Coagulation tube (light blue stopper, containing sodium citrate).
- Serum tubes (red, gold, or mottled; containing clot activators).
- Heparin tubes (green stopper, used for general chemistry tests).
- EDTA tubes (lavender or purple stopper).
- Glycolytic inhibitor tube (gray stopper, used for glucose and alcohol levels).
Adhering to this universally recognized protocol is necessary for maintaining the integrity of the blood samples and ensuring the accuracy of diagnostic testing.