Blood sample collection relies on a standardized system of tubes to ensure accurate test results. Each blood collection tube is distinguished by the color of its stopper, which indicates the specific additives contained inside. These additives preserve the blood sample in the exact state required for a particular analysis. Choosing the correct tube ensures the integrity of the sample, which is a prerequisite for reliable laboratory testing.
Identifying the Green Top Tube
The green top tube is readily identifiable by its stopper color and contains the anticoagulant Heparin. Heparin prevents the blood sample from clotting once it is drawn from the patient. This tube is designed to yield plasma, which is the liquid portion of blood that still contains clotting factors, rather than serum, which is the liquid remaining after the blood has clotted.
The Heparin is typically present as a salt, and the specific form used, such as Lithium Heparin, Sodium Heparin, or less commonly Ammonium Heparin, depends on the tests being ordered. Lithium Heparin is the most common choice for general chemistry testing, while Sodium Heparin is preferred for specific tests like flow cytometry or cytogenetics. The choice of salt is important because the ions in the additive can interfere with the measurement of the same ions already present in the patient’s blood. For example, Lithium Heparin is avoided when testing a patient’s lithium levels, and Sodium Heparin can artificially increase a patient’s measured sodium levels.
Primary Laboratory Applications
The green top tube is commonly used for a wide range of tests in clinical chemistry, particularly when rapid results are needed for patient care. It is a preferred container for “STAT” chemistry panels, which are ordered in emergency situations for a quick assessment of a patient’s metabolic state. These panels frequently include the comprehensive metabolic panel (CMP), providing information on kidney and liver function, glucose levels, and electrolytes.
Specific tests that rely on plasma from a green top tube include the measurement of ammonia levels, important for evaluating liver function, and certain cardiac markers like Troponin, used to diagnose heart attacks. The green tube is also the standard for blood gas analysis, which measures oxygen, carbon dioxide, and pH levels to assess a patient’s respiratory status and acid-base balance.
Why Heparin is the Preferred Anticoagulant
Heparin is employed in the green top tube because of its mechanism of action, which allows for the quick preparation of plasma. The anticoagulant works by binding to a naturally occurring plasma protein called antithrombin III. This binding causes a change in the shape of antithrombin III, which significantly increases its ability to inactivate several blood-clotting factors, most notably thrombin and Factor Xa.
By inhibiting thrombin, Heparin prevents the conversion of soluble fibrinogen into the insoluble fibrin mesh that forms a clot. This action keeps the blood sample in a liquid state, allowing the laboratory to immediately centrifuge the tube to separate the plasma from the cellular components. This contrasts with tubes that yield serum, such as the plain red or gold top tubes, which require a mandatory 15- to 30-minute period for the blood to fully clot before the serum can be separated.