Blood tests are a common part of healthcare, providing health insights. While samples are collected uniformly, their preparation for analysis differs significantly. This variation ensures specific components are isolated or preserved for various tests. Understanding these differences, especially regarding centrifugation, is important for accurate diagnostic results.
Understanding Centrifugation
Centrifugation is a laboratory technique that uses rapid spinning to separate fluid components, like blood, based on density. This process generates a strong centrifugal force, pushing denser particles to the bottom of the tube. In blood processing, this separates cellular components (red blood cells, white blood cells, platelets) from the less dense plasma or serum. This separation enables analysis of each component for medical tests and research.
Blood Tubes Requiring Centrifugation
Certain blood collection tubes contain additives that necessitate centrifugation to yield either serum or plasma for testing. These tubes are designed to prevent or promote clotting, depending on the required sample type.
Serum Tubes
Serum tubes, recognized by their red or gold (SST) tops, are used when serum is needed for analysis. Red-top tubes contain no additive or a clot activator, while gold-top tubes (SST) include a clot activator and a gel separator. The clot activator helps blood clot rapidly, within 15-60 minutes at room temperature. After clotting, centrifugation separates the serum (the liquid portion without clotting factors) from the clotted blood cells and the gel, which forms a barrier. Serum is used for chemistry panels, serology, immunology, and therapeutic drug monitoring.
Plasma Tubes
Plasma tubes, identifiable by their light green or green tops, contain anticoagulants like Lithium Heparin or Sodium Heparin. These anticoagulants inhibit clotting factors, allowing blood to remain liquid. Light blue-top tubes contain Sodium Citrate, another anticoagulant, specifically for coagulation studies. For these tubes, centrifugation separates plasma (the liquid portion containing clotting factors) from cellular components. Plasma is used for chemistry tests, including ammonia and troponin, and coagulation assays like Prothrombin Time (PT) and Partial Thromboplastin Time (PTT).
Blood Tubes Not Requiring Centrifugation
Some blood collection tubes are designed for tests that require the whole blood sample, meaning the cellular components are analyzed without separation from the liquid portion. These tubes contain anticoagulants that prevent clotting, allowing the blood to remain homogeneous.
Lavender or purple-top tubes
Lavender or purple-top tubes contain Ethylenediaminetetraacetic acid (EDTA), a strong anticoagulant that binds to calcium ions, preventing blood from clotting. EDTA is useful for preserving blood cell morphology and is used for hematology tests where cellular components are examined. Common tests from EDTA tubes include Complete Blood Count (CBC), blood typing, and hemoglobin A1c.
Black-top tube
The black-top tube contains Sodium Citrate, used for Erythrocyte Sedimentation Rate (ESR) tests. While Sodium Citrate is an anticoagulant, the ESR test measures the rate at which red blood cells settle in unclotted blood over a specific period, relying on the whole blood sample without centrifugation. In these cases, the integrity of cellular components within the whole blood is important for accurate results.
Impact of Proper Handling
Proper centrifugation techniques and adherence to specific protocols for each tube type are important for accurate test results. Incorrect handling, such as centrifuging a tube meant for whole blood analysis, can damage delicate cellular components, rendering the sample unsuitable. Similarly, failing to centrifuge a tube designed to yield serum or plasma would leave cellular components mixed, preventing accurate biochemical analysis. Such errors can lead to unreliable test results, potentially delaying diagnoses or requiring repeat blood collections. Adhering to standardized guidelines ensures sample integrity and contributes to patient safety.