Which additive is commonly used in blood tubes meant for coagulation tests?

Citrate is the commonly used additive in blood tubes designed for coagulation tests because it effectively chelates calcium, which is essential for the coagulation cascade. By binding to calcium ions in the blood sample, citrate inhibits the blood's ability to clot, allowing for accurate measurement of coagulation parameters such as prothrombin time (PT) and activated partial thromboplastin time (aPTT). This property is critical in ensuring that the results of these tests reflect the patient's coagulation status without interference from clot formation.

EDTA, while also an anticoagulant, is primarily used in hematology tests rather than for coagulation studies, as it can affect some coagulation test results. Heparin is another type of anticoagulant that is used in certain types of tests, but it is generally not preferred for routine coagulation testing due to its complex interactions with clotting factors. Gel separators are not anticoagulants; they contain a gel that separates serum or plasma from blood cells after centrifugation, but do not prevent coagulation in the initial blood collection. Thus, citrate stands out as the suitable choice for coagulation testing.