Ethylenediaminetetraacetic acid (EDTA) is a synthetic chemical widely used in medicine and science. This compound is a chelating agent, meaning it binds tightly to certain metal ions. In blood, EDTA acts as a preservative, ensuring samples collected for laboratory analysis remain in a liquid, unclotted state.
EDTA’s Role as an Anticoagulant
The most frequent application of EDTA in medicine is as a powerful anticoagulant in blood collection tubes, often identifiable by their lavender or purple tops. When blood is drawn, it naturally begins coagulation, or clotting, which renders the sample unusable for many tests. Preventing this response is necessary to maintain the sample’s integrity for accurate laboratory results.
EDTA is added to blood collection tubes to inhibit clotting immediately upon contact. Maintaining the blood in a whole, liquid state is crucial for tests that analyze cellular components, such as red blood cells, white blood cells, and platelets. Without EDTA, a clot would physically trap and alter the blood cells, leading to inaccurate counts and morphological analysis.
How EDTA Stops Blood Clotting
EDTA’s ability to halt coagulation stems from its chemical structure, which allows it to engage in chelation. Chelation involves the binding of EDTA to positively charged metal ions within the blood, particularly calcium ions ($\text{Ca}^{2+}$).
Calcium plays a fundamental role as a cofactor in the complex series of steps that make up the coagulation cascade. This cascade is a chain reaction involving numerous clotting factors that ultimately leads to the formation of a stable fibrin clot. By binding tightly to calcium ions, EDTA effectively removes them from circulation and makes them unavailable to the clotting factors. The removal of this ion prevents the activation of the cascade’s enzymatic reactions, thereby stopping the entire clotting process.
Specific Blood Tests Requiring EDTA
The preservation of whole blood integrity makes EDTA the anticoagulant of choice for diagnostic tests focused on cellular analysis. The most common test requiring EDTA-treated blood is the Complete Blood Count (CBC), which measures and analyzes the number and type of blood cells. For a CBC, the cells must be free-flowing and morphologically intact to allow for accurate automated counting and microscopic examination.
EDTA-treated blood is also essential for blood typing and crossmatching procedures performed before transfusions. These tests require red blood cells to remain intact and separate for proper testing against antibodies to determine compatibility. EDTA is also preferred for molecular diagnostic tests involving the analysis of DNA or RNA, as it helps stabilize nucleic acids and preserve cell morphology.
EDTA Use in Medical Therapy
Beyond its use as a laboratory preservative, EDTA has a distinct application as a therapeutic agent. This application is known as chelation therapy, where EDTA is administered intravenously to a patient. Here, the goal is not to preserve blood in vitro but to remove specific substances from the bloodstream in vivo.
The primary approved therapeutic use of EDTA chelation therapy is to treat acute heavy metal poisoning, such as from lead or mercury. When administered, EDTA circulates in the blood and binds to the toxic metal ions, forming a stable, water-soluble complex. This complex is processed by the kidneys and excreted through urine. The concentration and form of EDTA used in this therapy are carefully controlled and differentiated from its use as a blood sample anticoagulant.