Watching a phlebotomist fill multiple collection tubes during a blood draw can seem confusing or excessive. However, this practice is a standard and precise procedure governed by strict requirements. Drawing several vials is necessary to ensure the accuracy and reliability of diagnostic results. The need for multiple tubes stems from the diverse chemical preparations required for different types of laboratory analysis.
Why Different Tests Require Unique Tube Additives
Blood is a complex fluid composed of cells suspended in a liquid matrix, and laboratory tests analyze these components differently. Some procedures, such as blood typing, require the analysis of “whole blood” where all components remain mixed. For other tests, the liquid portion must be separated from cellular components like red and white blood cells. This separation is achieved by spinning the blood sample in a centrifuge after collection.
The type of liquid obtained after centrifugation depends on the chemical additives inside the collection tube. If the tube contains an anticoagulant, the blood will not clot, and the resulting liquid is called plasma. Plasma retains clotting factors, such as fibrinogen, and is often used for tests measuring drug levels or specific protein markers. Conversely, if the tube promotes clotting, the cellular components form a solid mass, and the remaining liquid is called serum.
Serum is blood plasma that lacks fibrinogen and other clotting proteins because they are consumed during the clotting process. Tubes designed to yield serum often contain a silica-based clot activator, which speeds up the natural coagulation cascade. This serum is primarily used for chemistry panels, immunology studies, and hormone level assessments. The absence of clotting factors is beneficial for these specific assays.
Decoding the Most Common Blood Tube Colors
The color of a collection tube’s rubber stopper is a standardized code that identifies the additive and the intended use of the sample. This system allows laboratory personnel to quickly sort and process samples correctly for the required test. The three vials frequently drawn represent the most common types of diagnostic panels ordered by a physician. Understanding these colors provides insight into the different types of testing being performed.
The first common tube is often gold, red, or tiger-striped, designed to produce serum for chemistry and immunochemistry testing. These tubes contain a gel separator and a clot activator, which speed up coagulation and separate the serum from the blood cells after spinning. This serum is used to measure electrolytes, liver enzymes, kidney function markers, and cholesterol levels. The clot activator, frequently silica particles, ensures the sample is fully clotted for accurate chemistry results.
A second tube frequently collected has a lavender or purple stopper, indicating the presence of the anticoagulant ethylenediaminetetraacetic acid (EDTA). EDTA works by binding calcium ions, which are required for blood coagulation, preserving the blood in its liquid whole-blood state. This whole blood sample is used primarily for hematology tests, such as the Complete Blood Count (CBC). Because EDTA preserves cell morphology, it is the standard for accurately assessing blood cell disorders and infections.
The third common vial is often light blue, which contains the anticoagulant sodium citrate. Sodium citrate prevents clotting by reversibly binding calcium, but its concentration is carefully calibrated to a specific blood-to-additive ratio. This tube is exclusively used for coagulation studies, which measure how quickly and effectively a patient’s blood clots, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT). Accurate diagnostic results for clotting disorders depend on maintaining this precise nine-to-one ratio of blood to sodium citrate.
The Critical Role of the Draw Order
The specific sequence in which tubes are filled, known as the Order of Draw, is a regulated procedural step. This order exists to prevent chemical additives from one tube from contaminating the blood sample collected in the next tube. Even minute amounts of an additive can significantly interfere with the assays performed on a subsequent sample.
For instance, the EDTA found in the purple-top tube can falsely lower calcium and magnesium levels if carried over into a chemistry tube, or it can invalidate coagulation results. Therefore, the light blue tube, which is sensitive to external additives, must be drawn before the purple EDTA tube. Following this protocol minimizes the risk of erroneous test results and ensures reliable diagnostic conclusions.