Blood collection tubes, often called vacutainers, are specialized, sterile containers designed to collect a precise volume of blood. These tubes utilize a vacuum seal and may contain chemical additives that prepare the blood for specific laboratory analyses.
Centrifugation is a physical process that separates blood components based on their differing densities. A centrifuge spins the collection tube at high speeds, generating a force that pushes heavier components to the bottom. This mechanical separation is fundamental when the test requires only the liquid portion of the blood, isolating it from the solid cellular material.
Understanding Blood Components
Diagnostic testing utilizes three primary forms of a blood sample: whole blood, plasma, and serum. Whole blood is the material as it is drawn, containing all the cellular elements suspended in the liquid matrix.
The need to separate the solid and liquid portions arises because many chemical and protein analyses require a cell-free sample. Plasma is the liquid portion obtained when whole blood is collected in a tube containing an anticoagulant, which prevents clotting. Centrifugation of this non-clotted sample separates the cells to the bottom, leaving the plasma layer on top.
Serum, conversely, is the liquid portion that remains after the blood has been allowed to clot naturally. To obtain serum, a sample collected without an anticoagulant is centrifuged, which separates the solid clot from the remaining liquid serum. Different laboratory tests are designed to be performed on either plasma or serum due to the differing protein content.
Tubes Requiring Separation
Tubes requiring centrifugation are those where the liquid portion (serum or plasma) is needed for analysis. These tubes contain additives that either encourage or prevent clotting, preparing the sample for separation.
Serum tubes are typically identified by a red, gold, or “tiger” top and contain a clot activator to speed up the natural clotting process. After clotting is complete, centrifugation forces the heavy blood cells and the resulting clot to the bottom. Many of these serum tubes also contain a gel separator, which forms a stable barrier between the clot and the serum after spinning.
Plasma tubes are characterized by various colors, such as green, light blue, or pearl white, and contain different types of anticoagulants. Green-topped tubes, for instance, contain heparin, which prevents coagulation and is often used for chemistry tests. Light blue tubes contain sodium citrate and are specifically used for coagulation studies.
Tubes Used for Whole Blood Analysis
Not all blood collection tubes are designed to be centrifuged, as some tests require the components to remain in their natural, suspended state. The most common tubes that should not be spun are those used for hematology studies, where the integrity of the blood cells is paramount for accurate cell counts and evaluations.
The lavender or purple-topped tube is the primary example of a whole blood analysis tube, containing the anticoagulant EDTA. EDTA works by binding to calcium, a mineral necessary for the coagulation cascade, thereby preventing the blood from clotting. This preservation of the whole blood sample is necessary for tests like a complete blood count (CBC), which measures the number and characteristics of red cells, white cells, and platelets.
Other whole blood tests that rely on the sample remaining unseparated include blood typing and some sedimentation rate analyses. Keeping the cellular components suspended and intact allows the laboratory instrument to accurately count and analyze the morphology of the blood cells directly.
Critical Factors in Sample Handling
The timing and technique of centrifugation are important to ensure the accuracy of laboratory results. For serum tubes, a sufficient amount of time, typically 30 to 60 minutes, must be allowed for the blood to fully clot before spinning. Centrifuging a serum sample too early can result in an incomplete clot, which may interfere with testing equipment later.
For both plasma and serum samples, separation from the cells should ideally occur within two hours of collection for many common chemistry tests. If the whole blood remains in contact with the cellular elements for too long, the cells continue their metabolic processes. This ongoing metabolism can cause changes in the fluid portion, such as a decrease in glucose levels and an increase in potassium, leading to inaccurate results.
Correct centrifugal force and time are also necessary to ensure a clean separation. Inadequate spinning can leave residual cells in the fluid, which can negatively impact the final analysis.