Interference of gadodiamide injection (OMNISCAN) on the colorimetric determination of serum calcium. 
The interference of the non-ionic magnetic resonance contrast medium gadodiamide injection (OMNISCAN, Nycomed Imaging, Oslo, Norway) in the colorimetric determination of serum calcium has been investigated in commercial reconstituted serum, and in serum from rabbits and humans dosed with the contrast medium.  Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ion-selective electrodes were used as reference methods for analysis of serum calcium.  The results showed that the colorimetric reagent kit gave an apparent decrease in serum calcium after administration of a clinical dose of gadodiamide injection, and that the extent of interference is correlated to the concentration of the contrast medium.  However, serum calcium was not changed when measured by means of an ion-selective electrode or ICP-AES.  It is therefore recommended that colorimetric reagent kits should not be used for determination of serum calcium in samples taken within the first 24 h after administration of gadodiamide injection.