title: Falsely elevated plasma selenium due to gadolinium contrast interference: a novel solution to a preanalytical problem.
abstract: BACKGROUND
  Trace elements are commonly measured by inductively coupled mass spectrometry (ICP-MS). A 30-year-old man had a plasma selenium (Se) concentration on ICP-MS of 66 µmol/L (reference interval 0.45-1.40), a potentially lethal level, despite no history of Se exposure or toxicity symptoms. He had earlier undergone magnetic resonance imaging with a gadolinium (Gd) contrast agent, which is known to interfere with Se on ICP-MS. We aimed to adjust our method by monitoring a second Se isotope that is unaffected by Gd to detect this preanalytical interference.
  
  
  METHODS
  Plasma samples referred for trace metal testing had Se measured on ICP-MS (monitoring (78)Se), which we modified to also monitor a second isotope ((82)Se). The modified method was then applied to a specimen with known Gd contamination.
  
  
  RESULTS
  Plasma Se results (n = 41) derived from monitoring the two different Se isotopes were similar with a good correlation (R (2 )= 0.991) over a range of 0.23-2.21 µmol/L. On repeat analysis, our patient had a Se concentration of 65 µmol/L using the (78)Se isotope but only 1.43 µmol/L using (82)Se.
  
  
  CONCLUSIONS
  To avoid reporting a falsely elevated plasma Se result, we suggest that Se analysis by ICP-MS should include a second Se isotope for monitoring, that is not subject to Gd interference.
authors.affiliation: Canterbury Health Laboratories, Christchurch, New Zealand Josh.Ryan@cdhb.health.nz.
authors.forename: Joshua B
authors.initials: JB
authors.lastname: Ryan
authors.affiliation: Canterbury Health Laboratories, Christchurch, New Zealand.
authors.forename: Susan
authors.initials: S
authors.lastname: Grant
authors.affiliation: Canterbury Health Laboratories, Christchurch, New Zealand.
authors.forename: Trevor
authors.initials: T
authors.lastname: Walmsley
authors.affiliation: Canterbury Health Laboratories, Christchurch, New Zealand.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Florkowski
authors.affiliation: Canterbury Health Laboratories, Christchurch, New Zealand.
authors.forename: Peter M
authors.initials: PM
authors.lastname: George
mesh_terms: D000328:Adult; D016477:Artifacts; D003287:Contrast Media / Q000737:chemistry; D005189:False Positive Reactions; D005682:Gadolinium / Q000097:blood*; D006801:Humans; D007554:Isotopes / Q000097:blood; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D013058:Mass Spectrometry / Q000379:methods*; D012643:Selenium / Q000097:blood*
chemical_list: D003287:Contrast Media; D007554:Isotopes; D005682:Gadolinium; D012643:Selenium
keywords: Selenium; assay interference; gadolinium; inductively coupled mass spectrometry; mass spectrometry; trace metals