Update on Selenium Testing: Shift from GSH‑Px to Whole Blood Selenium Assay

Due to an ongoing shortage of test kits, our usual method for assessing selenium status — glutathione peroxidase (GSH‑Px) — has not been available in recent weeks. Until supplies resume, we have temporarily replaced GSH‑Px with whole blood selenium analysis.

Why the Change?

Traditionally, GSH‑Px activity has been used as an indirect indicator of selenium intake, reflecting dietary selenium consumed approximately three weeks prior. While this remains a valuable measure, whole blood selenium is also a robust indicator of long‑term selenium status because it reflects both:

• The rapidly changing serum selenium pool, and
• The more slowly changing red blood cell (RBC) selenium pool

If only a red-top tube is available, serum selenium can still be performed. This reflects more recent dietary intake, though it does not account for the RBC fraction.

Reference Ranges

Our reported reference ranges are based on Herdt & Hoff, The Use of Blood Analysis to Evaluate Trace Mineral Status in Ruminant Livestock (Vet Clin North Am Food Anim Pract, 2011).

In addition, The Mineral Nutrition of Livestock (5th Ed.) provides the following marginal band guidelines:

Whole blood selenium:

• Adult cattle: 150–250 nmol/L
• Neonatal calves: 300–500 nmol/L
• Adult sheep: <500 nmol/L
• Lambs: <900 nmol/L

Serum selenium:

• Adult cattle: 100–120 nmol/L
• Neonatal calves: Not available
• Adult sheep: <250 nmol/L
• Lambs: <500 nmol/L

The closer the mean result is to the lower end of the marginal range, the more likely there will be health or production benefits from supplementation.

Clinical Considerations

Selenium plays an essential role in:

• Immune function
• Protection from peroxidative tissue damage

However, selenium deficiency is often multifactorial. Factors that can contribute to oxidative damage include:

• High polyunsaturated fatty acid diets
• Environmental temperature
• Exercise
• Vitamin E status

For this reason, clinical response to selenium supplementation is sometimes the most reliable indicator that low selenium was contributing to the clinical picture.

Additionally, liver selenium analysis in casualty animals can provide further diagnostic insight.