TOENAIL TRACE-ELEMENT LEVELS AS BIOMARKERS - REPRODUCIBILITY OVER A 6-YEAR PERIOD

We assessed the reproducibility over a 6-year period of 16 trace eleme nts measured in toenails by comparing levels in paired specimens colle cted in 1982-1983 and 1988 from 127 women in the United States. The Sp earman correlation coefficients for the reproducibility of toenail lev els of selenium and arsenic (both known to reflect intake of these ele ments) were 0.48 and 0.54. Correlations for other elements ranged from 0.26 (copper) to 0.58 (zinc). In utilizing biomarkers to assess expos ure in epidemiological studies of cancer and other chronic diseases, r andom within-person variability in exposure leads to attenuation of me asures of association between exposure and disease. We demonstrate the effect of such variability on odds ratios from a hypothetical case-co ntrol study. For a true odds ratio of 3.0 (for a comparison of the hig hest quintile versus the remaining 4 quintiles of exposure) the odds r atios which would be observed in the presence of the degree of within- person variability demonstrated in this study were 2.15 for toenail ar senic and 1.67 for toenail copper levels. Toenail concentrations of ce rtain trace elements are useful biomarkers of exposure in which a sing le sample is assumed to represent long-term exposure. However, substan tial attenuation in measures of association may occur.