The invention relates to a process for preparing a liquid pharmaceutical formulation containing a complex of macrocyclic chelate with a lanthanide and a mol/mol amount of free macrocyclic chelate of between 0.002% and 0.4%, advantageously between 0.02% and 0.3% and very advantageously between 0.025% and 0.25%, the macrocyclic chelate advantageously being chosen from DOTA, NOTA, DOTAGA, DO3A, BT-DO3A, HP-DO3A and PCTA, and is preferably DOTA, the said process comprising the following successive steps: b) preparation of a liquid pharmaceutical composition containing, firstly, the complex of macrocyclic chelate with a lanthanide, and, secondly, free macrocyclic chelate and/or free lanthanide; c) measurement in the pharmaceutical formulation obtained in step b) of the concentration of free macrocyclic chelate Cch1 and/or of free lanthanide Clan1; d) adjustment of Cch1 and/or of Clan1 so as to obtain Cch1=Ct ch1 and Clan1=0, wherein Ct ch1 is the target concentration of free macrocyclic chelate in the final liquid pharmaceutical formulation.