Abstract
Selenium is an important clinical biomarker of nutritional status; however, the occurrence of gadolinium in a patient's serum as a result of the contrast agents used during magnetic resonance imaging investigations, results in a significant positive bias in its measurement by inductively coupled plasma-quadrupole mass spectrometry.
Introduction
Selenium is an important essential trace element. 1 It is incorporated into a number of enzymes with metabolic functions such as the synthesis of thyroid hormones and the protection of cell membranes against reactive oxygen species. Selenium also guards against development of some cancers, and a reduced incidence of prostate cancer in men taking dietary supplements has been reported. 2 Deficiency of Se may be seen if there is inadequate intake in the diet, if there is poor intestinal absorption and in patients who are on prolonged intravenous nutrition. Selenium is also toxic if there is excessive exposure. This occurs in some parts of the world where the soil, and hence crops, contain naturally high concentrations of this element. More usually, toxicity is associated with occupational exposure or in incidents of deliberate poisoning. 1
Case report
Diagnosis of deficiency or toxicity is made by measuring the Se concentration in serum or whole blood. In October 2008, a serum sample from a patient with chronic fatigue syndrome was referred to the Guildford Trace Element Laboratory for measurement of Se. Although not stated on the form that accompanied the specimen, it was thought that the reason for the investigation was either that she may be Se-deficient with an associated deficiency of thyroxine or that she was consuming an inadequate diet. The sample was analysed using a Thermo Scientific X SERIES inductively coupled plasma-quadrupole mass spectrometer (ICP-QMS) equipped with a collision cell. Samples, internal quality control materials, reagent blank and aqueous calibration solutions with concentrations of 0.5–4.0 μmol/L, were diluted 50-fold by the addition of 200 μL to 9.65 mL 0.5% (v/v) HNO3 and 150 μL internal standard solution (100 μg/L Ge). The instrumental conditions established for the measurement are shown in Table 1.
Instrument conditions used to determine Se in serum with Thermo Scientific X-Series ICP-QMS
ICP-QMS, inductively coupled plasma-quadrupole mass spectrometry
The Se result was 16.8 μmol/L, compared with the reference range of 0.89–1.65 μmol/L for humans. 3 This extraordinarily high concentration would normally be seen only after a massive exposure and would be expected to be accompanied by symptoms of Se poisoning.
A repeat sample for the patient was requested from the hospital and the original specimen was re-measured using Zeeman graphite furnace atomic absorption spectrometry (GFAAS). The second specimen, taken seven days after the first, gave a result of 1.06 μmol/L by ICP-QMS and the re-analysed first sample result was 0.65 μmol/L by GFAAS. The patient showed no symptoms of Se toxicity. Brittle hair and/or nails were not evident and there was no garlic-like odour.
Further investigations and discussion
It was concluded that an interference was affecting the ICP-QMS measurement. Some interferences are known to occur with ICP-QMS but, to our knowledge, an effect of this magnitude on the measurement of Se in human samples has not been previously reported. In looking for a possible explanation, it was noted that 156Gd2+ would cause an interference on 78Se+, the isotope used for the selenium measurement, because quadrupole mass spectrometers separate isotopes prior to detection on the basis of their mass-to-charge ratio.
While most gadolinium is present within the ICP as 156Gd+, there is a proportion that becomes doubly charged Gd2+, and therefore has a mass-to-charge ratio of 78. The original sample was re-examined and while the concentration of Gd was not quantified, the mass spectrometer counts at each of the Gd isotopes were several million, indicating that a very large amount of the element was present. Samples of serum would normally have undetectable concentrations of Gd. The referring hospital subsequently confirmed that the patient had had a magnetic resonance imaging (MRI) scan using a contrast agent containing Gd, on the same day as the sample was taken for the Se measurement. Approximately six months later, a second example of this interference was seen with an apparent serum Se concentration of 9.7 μmol/L in a subject who was being assessed in connection with an intestinal transplant. He too had previously had an MRI scan using a Gd contrast agent.
To establish the degree of interference from doubly charged Gd ions, a human serum sample was spiked so as to contain 0.2, 0.5, 1.0, 2.0 and 5.0 mg/L Gd and the Se concentration measured using the 78Se isotope. The results are shown in Table 2. The ICP conditions were established so as to generate <3% doubly charged ions. It is clear from the data that under these conditions, concentrations of between 1 and 2 mg/L Gd present in the serum sample would produce a Se signal covering the human reference range for Se. The clearance of Gd following its use as a contrast agent has been investigated in healthy subjects 4 and in patients with impaired renal 5 or liver 4,6 function. Following an intravenous injection of 0.2 mmol (31.2 mg) Gd per kg body weight to healthy volunteers, peak plasma concentrations immediately after dosing were approximately 1 mmol/L (157 mg/L). 4 Concentrations were followed for 12 h and showed a linear decay to 0.01 mmol/L (1.57 mg/L). Measurements were not continued beyond this time. Further work is being undertaken to investigate possible procedures that will demonstrate the Gd interference, such as measurement of the 78Se, together with either Gd or an unaffected Se isotope, so that inaccurate results will not be reported.
Investigation of the contribution of Gd2+ to the apparent Se concentration, as measured by ICP-QMS, in a human serum sample
IQC, internal quality control; UKNEQAS, UK NEQAS for Trace Elements, Guildford, Surrey; ICP-QMS, inductively coupled plasma-quadrupole mass spectrometry
Each result is the mean of three replicate measurements
Conclusion
We have demonstrated that Gd, present in human serum samples following the use of Gd-contrast agents for MRI, causes a positive interference on the measurement of Se by ICP-QMS. Interference was observed, in vitro, with Gd concentrations as low as 0.2 mg/L. The time taken for the Gd to be eliminated from the circulation to concentrations lower than this is not known. With increasing use of Gd-contrast agents for both MRI and other radiological investigations, 7 and with more laboratories using ICP-QMS rather than GFAAS, it would be anticipated that similar incidents will occur. Indeed, we are aware of further examples that have been seen in other laboratories. Interference to the extent reported here should not be a clinical problem as the apparent Se concentrations are so high that results will not be accepted as genuine. The concern lies with situations where the interference is much less but causes a low Se concentration to appear within the reference range, and for Se deficiency to be undetected.
DECLARATIONS