EDTA salts (ethylenediaminetetraacetic acid) are used to anticoagulate whole blood for haematological investigations, as the cellular components of the blood are particularly well preserved by EDTA. The anticoagulation is achieved by the EDTA forming complexes with metal ions such as calcium, therefore inhibiting the coagulation cascade. 
Anticoagulation with EDTA is irreversible.
To avoid micro-clotting, mix the EDTA tube with 8-10 inversions immediately after the blood sample has been taken.