Selenium

This element has attracted great interest as an essential element; of recent interest is the prevention of certain cancers by supplementation with selenium.

The soil in parts of New Zealand, China and Finland are notably selenium deficient and this is reflected in lower selenium concentrations in blood and urine. An unusual form of cardiomyopathy (Keshan Disease) was found to be endemic in an area of China and has now been prevented by prophylactic administration of supplemental selenite.

Patients receiving supplemental nutrition with highly purified synthetic nutrients, either orally or during total parenteral nutrition (TPN), can become selenium depleted. Only rarely does this cause overt clinical signs and symptoms such as muscle weakness and pain and cardiomyopathy.

Selenocysteine is the biologically active form and is involved in the active site of several enzymes involved in oxidation-reduction reactions. There is persuasive evidence suggesting that selenium above normal UK reference intervals helps protect against oxidant stress, improves immuno-responsiveness and lowers overall risk of cancer.

Selenium status can be assessed by measurement of plasma selenium which responds to changes in intake. It is, however, subject to an acute phase response which lowers the concentration by up to 60% immediately following a trauma. Measurement of a selenoprotein, glutathione peroxidase (GPX), provides an alternative functional test for selenium status. It responds more slowly than selenium to changes in intake and is not subject to an acute phase response.

In the absence of guidelines on when patients should be supplemented, we suggest this is appropriate in adults with plasma selenium of less than 0.6 µmol/L and GPX less than 10 IU/g haemoglobin.

In man, selenium toxicity presents as skin eruptions, gastrointestinal upset, hair and nail changes, and discoloration of teeth. An increased incidence of dental caries is also reported.

Sample Requirements and Reference Ranges for Selenium

Sample Type Plasma or serum
Container Lithium heparin, plain, or ‘Trace Metal’
Precautions None
Minimum volume* 250 µL
Reference ranges 0.8 – 2.0 µmol/L (adult)

0.2 – 0.9 µmol/L (0 to 2 years)
0.5 – 1.3 µmol/L (2 to 4 years)
0.7 – 1.7 µmol/L (4 to 16 years)

> 2.5 µmol/L (in adult) possible toxicity

Turnaround time 1 week
Method Inductively coupled plasma mass spectrometry

Sample Requirements and Reference Ranges for GPX

Sample Type Whole blood
Container Heparin, EDTA, Trace Metal tube
Precautions None
Minimum volume* 150 µL
Reference range 15 to 50 IU/g haemoglobin
Turnaround time 2 weeks
Method Enzymatic

* This is the absolute minimum volume; these volumes are insufficient to carry out a repeat analysis in the event of an analytical problem.