An anticoagulant is a preservative that used to preserve the blood sample in a state similar to its state inside the body. An anticoagulant that is used for cbc must be neither affecting the structure of the formed elements of blood, nor produce any dilution to the plasma.
Ethylene Diamine Tetra-acetic acid (EDTA) is considered the anticoagulant of choice for cbc, taking in consideration that EDTA must be used in the proper concentration (1mg/ml blood). In case of blood collected in a vacutainer tube, the proper amount of blood must be collected till the mark indicated on the tube to avoid errors related to the increased concentration of EDTA outside the cells (collection of blood below the mark), which may result in shrinkage of RBCs.