Gadolinium-based contrast agents (CAs) are widely used to enhance the contrast of images in magnetic resonance imaging procedures. Two categories of gadolinium chelates exist: the macrocyclic molecules where Gd3+ is caged in the pre-organized cavity of the ligand and the linear molecules. Gadolinium chelates differ in their thermodynamic stability constants and in their kinetic stability. In general, macrocyclic chelates such as Gd-DOTA or Gd-HP-DO3A are more stable than linear molecules. Even among linear agents, differences can be found. There is increasing evidence that transmetallation can be found in vivo, in the case of certain CAs (especially linear chelates), with body cations such as zinc, calcium or iron. Furthermore, analytical interference with colorimetric determination of calcium has been clinically evidenced with two linear chelates, Gd-DTPA-BMA and Gd-DTPA-BMEA. Clinical cases of spurious hypocalcaemia have been reported with these molecules. Such interference with some colorimetric assays for calcium is clinically relevant in that it can lead to unnecessary and potentially harmful treatment for hypocalcaemia.