Additive: K2EDTA or K3EDTA.
K2EDTA (disodium ethylenediaminetetraacetate disulfate) contains two sodium ions and one disulfate ion.
K3EDTA (tripotassium ethylenediaminetetraacetate) contains three potassium ions.
EDTA is a chelator that binds calcium ions, which are necessary for blood clotting. Once the blood is in the EDTA tube, it is kept in a stable state without clotting.