Serum and plasma selenium responds to short-term changes in diet and measuring the amounts of selenium in serum and plasma can indicate short-term selenium status. Normal selenium levels of the U.S. population fall within the range of 2.03-3.29 micromoles (mcmol) per liter (L).
Selenium levels in whole blood do not fluctuate much and are thus more indicative of long-term selenium status. A new technique that can be used to analyze serum, plasma or whole blood selenium is called graphite furnace atomic absorption spectrometry. This method is fairly easy to execute and the resultant samples require little pretreatment.
Urine samples reflect selenium content of the previous meal and are not useful for assessment of selenium status, though they can be used to detect toxicity.
Toenail selenium is useful for assessment of long-term selenium status. Selenium is incorporated into toenails as they grow. Because the exposed toenail on which tests are done have long since ceased to grow, toenail selenium levels probably reflect zinc status as it stood six to nine months prior to testing.
There is a correlation between blood glutathione peroxidase activity and blood selenium up to 1.2 mcmol/L. Beyond that point the activity plateaus, and cannot be used to determine selenium status. The test is not specific because other factors can affect glutathione peroxidase activity. Such factors include age, sex, ethnicity, iron-deficiency anemia, essential fatty acid deficiency, and vitamin B12 deficiency.