Phlebotomy relies on a standardized system of color-coded vacuum tubes. Each color indicates the presence of a specific chemical additive, or lack thereof, designed to preserve the blood sample in the required state for testing. This color coding ensures that the correct tube is used for the intended diagnostic examination. The purple-top tube, often referred to as the lavender-top tube, is frequently utilized in a clinical setting. Its distinctive color signals that the blood collected inside is meant for tests requiring the cellular components to remain in their natural, whole-blood state.
The Critical Additive: EDTA
The substance responsible for the purple tube’s unique function is Ethylenediaminetetraacetic acid, commonly abbreviated as EDTA. This chemical acts as a powerful anticoagulant, meaning it prevents the blood from clotting after it has been drawn from the patient. The goal of using EDTA is to keep the blood sample entirely liquid, preserving it as whole blood for accurate cell counting and analysis. EDTA is typically present as a potassium salt, most often in the form of dipotassium EDTA (K2 EDTA) or tripotassium EDTA (K3 EDTA). K2 EDTA is often preferred for its minimal effect on red blood cell size.
How EDTA Prevents Coagulation
The mechanism by which EDTA stops blood from clotting is a process called chelation. Blood coagulation requires a complex chain reaction known as the coagulation cascade, which cannot be initiated without the presence of free calcium ions (Ca²⁺). EDTA is a chelating agent, a molecule structured to bind tightly to metal ions, including calcium. Once the blood enters the purple tube, the EDTA immediately sequesters the calcium ions. By effectively removing the calcium from the blood plasma, EDTA interrupts the entire clotting cascade. This action prevents the formation of fibrin, which is the protein mesh that forms the structure of a blood clot. The resulting sample is preserved with all its cellular components—red cells, white cells, and platelets—intact and dispersed for precise laboratory measurement.
Key Diagnostic Tests Requiring the Purple Tube
The ability of the purple tube to provide an uncoagulated whole blood sample makes it indispensable for hematology and other specialized tests. The most frequent use is for the Complete Blood Count, or CBC, which is a broad screening test that measures the number of red blood cells, white blood cells, and platelets in the blood. Since the CBC determines the concentration and morphology of these cells, it requires them to be unaltered by the clotting process. The purple tube is also the standard for hemoglobin A1C testing, which measures average blood sugar levels over several months by examining glycated hemoglobin in red blood cells. Blood typing and cross-matching for safe blood transfusions also rely on EDTA-preserved samples, as the cellular integrity is vital for accurate antigen and antibody testing. Finally, the tube is used for the preparation of a peripheral blood smear, where a thin layer of blood is examined under a microscope to evaluate the shape and health of the blood cells for diagnosing conditions like anemia or leukemia.