Selenium is an element necessary for basic cellular function that can be found in nuts, fish and shellfish, meat, eggs and whole grains. Too little of this mineral can produce thyroid problems, weakening of the heart and potentially fatal diseases, but deficiencies aren't common in healthy people with a normal diet. Most people with low selenium levels in their blood live in areas where the soil and plants grown in it are deficient in this nutrient. People with compromised gastro-intestinal functions, including the very old, people with intestinal diseases and patients who have had gastrointestinal bypass surgery are also at risk of low selenium levels.

For most of the population, selenium serum tests are performed to determine whether selenium poisoning is a risk. Exceeding exposure of more than 400 micrograms per day can result in selenosis, which produces garlicky-smelling breath, digestive problems, fatigue and hair loss. Over time, it can produce neurological damage, liver cirrhosis, edema or even death. Workers in some industrial jobs and people in areas where the water has been contaminated by agricultural runoff are at the greatest risk of a selenium overdose, but may not know they have the problem in its early stages.

A selenium serum test can detect excessively low or high levels of selenium in the body, allowing patients to adjust their diet or environment to correct the problem. This test requires no fasting before blood is drawn, but specimens must be collected carefully since the mineral is volatile. Most people should have test results of between 70 and 150 nanograms per milliliter.