MINERALS AND TRACE ELEMENTS
A diagnosis of an overt mineral deficiency or excess can be confirmed by chemical tests that measure its concentration in the serum or, in some cases, urine. These measurements can give an indication of the amounts present in body tissues, although urine values tend to reflect dietary intake rather than the amounts stored in the body. The value of determining the serum concentrations of sodium, potassium, calcium and phosphate have been considered. Investigations of chloride and sulfate are of little clinical relevance.
Measuring the concentrations of trace elements in clinical samples is complex and requires sensitive techniques because of their low values. Samples of plasma are often used but the determined values may not accurately reflect the concentration of the trace element at its site of action, which may be intracellular. However, for many trace elements, a low plasma concentration is indicative of a deficiency and adequate supplementation needs to be provided, while a high value is an indicator of possible toxic levels. Copper deficiency is uncommon, except in patients on synthetic oral or intravenous diets. In these patients, serum copper is reduced to less than 12 Lmol dm–3. Low concentrations of plasma copper may indicate depleted stores but are a poor indicator of short-term copper status. Measuring Cu/Zn superoxide dismutase and cytochrome oxidase activities can also indicate copper status as the activities of these enzymes are reduced on a low copper diet. There is no satisfactory test for chromium deficiency and a diagnosis is usually made following improved glucose tolerance after chromium supplementation. Serum fluoride concentration can indicate fluoride exposure and can provide information on endemic fluorosis and allow preventative measures to be taken. Some, but not all studies, have reported a direct relationship between serum fluoride and the degree of fluorosis. Measurements of serum iodine may be a useful assessment of thyroid activity in adults. They may also be of use in investigating cretinism in infants, allowing a diagnosis to be made at an earlier age than is possible by other methods. Serum iodine measurements are also of value in assessing iodine toxicity. Methods to investigate the iron status of a patient are described. Serum iron and total iron binding capacity can be investigated but serum ferritin is a better measure of total body iron stores. The amount of magnesium in serum is less than 1% of the total in the body and is therefore a relatively poor indicator of magnesium status. If hypomagnesemia is present, then magnesium deficiency is likely but a normal serum value does notexclude a significant deficiency. Measurements of molybdenum in biological fluids are rarely required, which is perhaps fortunate since the methods used are inadequate due to the low concentrations involved. Selenium deficiency occurs with poor dietary intake and can be detected by its measurement in plasma or whole blood. However, determining erythrocyte activity of the selenium dependent enzyme, glutathione peroxidase can indirectly assess selenium status. Zinc concentrations less than 8 Lmol dm–3 in the plasma may indicate zinc deficiency but low values may be associated with hypoalbuminemia, as most zinc is bound to albumin.Chemical tests for mineral and trace element deficiencies must always be used to complement the medical history and physical examination since many of their findings may reflect underlying disease additional to the nutritional status of a patient. It is therefore necessary to understand these illnesses and how they influence the findings of physical and laboratory investigations.