Falsely elevated plasma selenium due to gadolinium contrast interference: a novel solution to a pre-analytical problem

Abstract

Aims/Background Trace elements are commonly measured by inductively coupled mass spectrometry (ICP-MS). A 33-year-old male had a reduced plasma zinc on ICP-MS of 8.5 μmol/L (reference interval 10-17). Plasma selenium (Se) concentration was incidentally found to be 66 μmol/L (reference interval 0.45-1.4), a potentially lethal level, despite not taking selenium supplements, no occupational exposure and no toxicity symptoms. He had earlier undergone magnetic resonance imaging (MRI) with gadolinium (Gd) contrast which is known to interfere with Se on ICP-MS (as the mass to charge ratio of Gd2+ with atomic weight 156 is the same as two Se+ with atomic weight 78). We aimed to adjust our method to prevent this pre-analytical interference. Methods Selenium is measured on an Agilent 7700 (by monitoring Se-78 isotope), which we modified to also include the Se-82 isotope. Results Plasma Se results (n = 40) derived from monitoring the two different Se isotopes gave similar results with a good correlation (R = 0.99). On repeat analysis, our patient had Se concentration of 65 μmol/L with the Se-78 isotope but only 1.43 μmol/L using Se-82. Gd-156 isotope was also elevated. Conclusion To avoid misleading Se results, we suggest that plasma Se analysis by ICP-MS should include a second Se isotope that is not subject to this interference.