COMPARISONS OF NAIL MEASUREMENT TO MEASUREMENTS IN OTHER TISSUES
In addition to nail, trace element assessments in blood, hair and urine specimens have been used in clinical studies. Although comparisons among these markers may not help to determine the accuracy of the measurement of an individual marker since none of them can serve as a gold-standard for others, the measurement in concordance with others may support the capacity of the marker in clinical research. A few studies have been conducted comparing measurements in nail, blood, hair and urine. In general, each of the biomarkers is considered effective for particular clinical studies.39–41 However, compared with blood or urine sample, nails particularly toenails provide a relatively long-term measure of exposure. For example, Arsenic can appear initially throughout the body, but it clears rapidly from most tissues, including the blood stream. After methylation in the liver, it is excreted in the urine;42 as a result, blood and urine concentrations reflect only relatively recent exposure.43 Also, nail clippings are non-invasive and relatively easy to collect especially can be completely self-administered, and easy stored at room temperature. Human hair is formed of the same keratinous tissue of nail and is therefore expected to share some advantages of nail marker. However, hair analysis is prone to be affected by cosmetic procedures such as dyeing, bleaching, and permanent waving which alter trace element content in hair.44 In fact, hair as a biomarker for assessing trace element status has been questioned.45–48 Although both finger- and toe-nail can serve as biomarkers for trace element status, toenails generally provide a larger sample and represent exposures in the more distant past because they take longer to grow out. Overall, nails provide valid measurements for ranking subjects according to long-term trace element intake or exposure.