Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1299 page)

   Inflammatory bowel disease

   Rickets

   Alzheimer disease

   Limitations

   More recently, it has become clear that receptors for vitamin D are present in a wide variety of cells and that this hormone has biologic effects extending beyond the control of mineral metabolism.

   Vitamin D deficiency is not clear. Levels needed to prevent rickets and osteomalacia (15 ng/mL) are lower than those that dramatically suppress parathyroid hormone levels (20–30 ng/mL). In turn, those levels are lower than levels needed to optimize intestinal calcium absorption (34 ng/mL). Neuromuscular peak performance is associated with levels approximately 38 ng/mL. A recent study states that increasing mean baseline levels from 29 to 38 ng/mL was associated with a 50% lower risk for colon cancer and levels of 52 ng/mL with a 50% reduction in the incidence of breast cancer.

   Various methods for measuring circulating concentrations 25-OHD are available. Current methods include RIA, CIA, HPLC, and LCMS/MS tandem mass spectrometry. Immunoassays measure total 25-OHD, which includes levels of both 25-OHD
₂
and 25-OHD
₃
. The antibodies crossreact 100% with both D
₂
and D
₃
to give the total 25-OHD. Some commercial laboratories use LCMS/MS technology and report 25-OHD
₂
and 25-OHD
₃
separately and add both values to get the total 25-OHD. The studies report reasonable correlations between methods, but with significant differences, the reasons for which are not well understood. There could be many reasons for these variations, including drifts in the reagents being manufactured, and there is an urgent need for harmonization and standardization.

   The reference ranges discussed in the preceding are related to total 25-OHD; as long as the combined total is 30 ng/mL or more, the patient has sufficient vitamin D. The Institute of Medicine 3, 4 and the Endocrine Society 5 announced that levels <20 ng/mL (50 nmol/L) are considered deficient, which is lower than in previous guidelines. Given the absence of assay standardization and lack of consensus regarding clinical cutoff values, vitamin D levels must be interpreted within the clinical context of each patient and one should not rely solely on cutoff values based on so-called normal values.

VITAMIN E (ALPHA-TOCOPHEROL)

   Definition

   Tocopherol is a fat-soluble vitamin with antioxidant properties; it protects cell membranes from oxidation and destruction. Vitamin E is found in a variety of foods, including oils, meat, eggs, and leafy vegetables. Serum vitamin E levels are strongly influenced by concentration of serum lipids and do not accurately reflect tissue vitamin levels. Effective vitamin E levels are calculated as the ratio of serum alpha-tocopherol per gram total lipids. Vitamin E reserves in lung tissue provide a barrier against air pollution and protect red blood cell membrane integrity from oxidation. Oxidation of fatty acids in red blood cell membranes can result in irreversible membrane damage and hemolysis. Studies are in progress to confirm the suspicion that oxidation also contributes to the formation of cataracts and macular degeneration of the retina. Because vitamin E is found in a wide variety of foods, a deficiency secondary to inadequate dietary intake is rare.

   
Normal range:
see Table 16.85.