Gadolinium species or speciation refers to the molecular structure Gadolinium is in. Gadolinium administered as a chelate (GBCA) to an individual remains either in its chelated form (intact chelate) or dechelated or disassociated form. Gadolinium is essentially never 'free', because if Gd dechelates from the GBCA, it extremely rapidly recombines with other molecules. The most common other species that Gd appears as in the body are as a salt combined with carbonates, phosphates, or similar molecules, or attached to a proteinaceous macromolecule. With macrocyclic GBCAs, the majority, near entirety, of the Gd will be in its chelated form as the injected GBCA. Linear agents, depending on their thermodynamic stability, will have varying amounts of Gd in the salt or macromolecular forms. The less stable the GBCA, the more disassociated amounts of Gd will be present, as salts or protein macromolecules.
It is not clear how important the species that Gd is present in, is for the development of GDD, as macrocyclic agents also cause GDD, and the GBCAs will be intact when the disease initiates. It is also not certain if the speciation that occurs in 'normal' GSC patients (all the patients who have been studied so far), is the same in GDD patients. Specific examination of GDD patients is also needed.