What is the difference between the use of «citrate» and «EDTA» as an anticoagulant in medicine (I know that each one is used for some dosages but not the other, but I need to know why)?
I'm not familiar with using citrate or EDTA as an anticoagulant in medical care (i.e. as a drug). However, it is often used in blood sampling tubes so that the blood sample does not clot.
As to their difference, EDTA works irreversibly while citrate is reversible. EDTA is used more often most notably to get the complete blood count. Citrate is mostly used to assess the bloods ability to clot (by reversing the anticoagulation). In rare cases we need to use citrate tubes to assess platelet numbers since EDTA makes the platelets cling together, this is called EDTA agglutination.
Regarding the mechanisms behind the anticoagulation, EDTA irreversibly binds calcium ions which are essential for many enzymes in the coagulation cascade. Citrate also binds calcium ions but also seems to affect other parts of the coagulation cascade.
Banfi et. al. (2007)The role of ethylenediamine tetraacetic acid (EDTA) as in vitro anticoagulant for diagnostic purposes. Clin Chem & Lab Med; 45(5): p565-76
Mann et. al. (2007) Citrate anticoagulation and the dynamics of thrombin generation.J ThrombHaemost;5: 2055–61