Phlebotomy involves drawing blood for various medical tests, a procedure that demands considerable precision. Accurate blood collection is foundational for reliable diagnostic results, directly influencing patient care decisions. A protocol governing this process is the “order of draw,” a specific sequence in which blood collection tubes must be filled. Adhering to this sequence is standard practice designed to maintain sample integrity and prevent errors that could compromise test accuracy.
Overview of Blood Collection Tubes
Blood collection relies on various tubes, each designed for specific laboratory tests and identifiable by the color of its stopper. These tubes often contain different chemical additives that serve particular functions. For instance, some tubes contain anticoagulants, substances that prevent blood from clotting, allowing for whole blood analysis or plasma separation.
Other tubes feature clot activators, which promote rapid blood clotting to yield serum for biochemical tests. Tubes designed for trace element analysis may have no additive at all, or a specific inert material. Understanding these additives’ functions is important for their proper handling and sequence during collection.
The Standard Sequence for Drawing Blood
A standardized sequence for drawing blood samples has been established to ensure consistency and prevent contamination. The order is as follows:
- Blood culture bottles: Collected first to minimize microbial contamination.
- Light blue stopper tubes: Contain sodium citrate, an anticoagulant for coagulation studies.
- Serum tubes (red or gold stoppers): Contain clot activators or no additive, allowing blood to clot for serum.
- Green-stoppered tubes: Contain heparin, an anticoagulant for chemistry tests.
- Lavender-stoppered tubes: Contain EDTA, an anticoagulant for hematology tests.
- Gray-stoppered tubes: Contain sodium fluoride and potassium oxalate, used for glucose measurements.
How Additive Carryover Compromises Samples
The meticulous order of draw is implemented to prevent additive carryover, a phenomenon where even microscopic amounts of an additive from one tube can contaminate the next. This contamination can significantly alter the chemical composition of the subsequent blood sample, leading to inaccurate test results.
For example, if an EDTA tube (lavender stopper) is drawn before a tube for coagulation studies (light blue stopper), trace amounts of EDTA can be carried over. EDTA functions by binding calcium, which is necessary for blood clotting, thereby interfering with coagulation tests like Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), making the blood appear to take longer to clot.
Similarly, heparin, found in green-stoppered tubes, is a potent anticoagulant. Carryover of heparin into tubes intended for blood cell morphology studies can interfere with cell staining characteristics or cause cell clumping, making microscopic examination difficult or misleading.
Clot activators present in serum tubes (red or gold stoppers) can cause micro-clots if carried over into tubes intended for whole blood analysis, such as those for a complete blood count. These micro-clots can lead to falsely low cell counts or abnormal cellular indices, as automated analyzers may not detect all cells trapped within the small clots.
Consequences for Patient Diagnosis and Care
Incorrect test results, often stemming from additive carryover due to an improper order of draw, can have profound consequences for patient diagnosis and care. A misdiagnosis or a delayed diagnosis might occur if laboratory values are inaccurately high or low.
For instance, falsely abnormal electrolyte levels due to additive interference could lead a physician to pursue unnecessary diagnostic tests or prescribe incorrect medications. Such inaccuracies can also result in inappropriate or delayed treatment, potentially worsening a patient’s condition or prolonging their illness.
Inaccurate results frequently necessitate repeated blood draws, subjecting patients to additional discomfort, anxiety, and the potential for vein damage. This also increases healthcare costs due to re-testing, additional laboratory reagents, and potentially extended hospital stays, underscoring the broad impact of adhering to the standardized order of draw in phlebotomy.