What is the purpose of citrate in blood collection for coagulation testing?

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Calcium is a required cofactor at several steps of the coagulation cascade. Citrate in collection tubes binds calcium ions, removing free Ca2+ from the blood and thereby stopping clot formation. This keeps the sample fluid for accurate coagulation testing. The binding is reversible, so calcium can be added back during the assay to allow clotting measurements to proceed. Citrate does not activate clotting factors, does not release calcium, and does not primarily affect magnesium.