Reliability of toenails as biomarkers of chronic metal exposure in the US Gulf States

Background/Aim: Toenails are a promising matrix to assess chronic metal exposure, but questions remain about the sample mass requirements and temporal variability of metals measured in this matrix. This study evaluates the reliability of a ~25 mg toenail sample (1 - 2 clippings) for metals analysis and the temporal variability of the toenail biomarker over 2 - 4 years in a cohort of men from the Gulf Long-term Follow-up (GuLF) Study. Methods: Toenail samples from 106 GuLF Study participants were collected at two visits 2 - 4 years apart and analyzed for 18 metals (Al, As, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Sb, Se, V, Zn) using inductively coupled mass spectrometry (ICP-MS). Participants with samples >120 mg (n=24) were selected for 3x repeated analysis using ~25 mg sub-samples. Intraindividual correlation coefficients (ICC) were used to assess the reliability of the sub-sample mass. Linear mixed models were used to assess toenail metal correlations over time. Results: ICCs and correlations over time were not reported for Cd, Co, Mo, Sb, and V which were detected in less than 60% of the samples. ICCs for the remaining metals ranged from 0.48 for Fe to 0.88 for Cu. Correlations of metal concentrations over time ranged from 0.06 to 0.66 with Ca (0.06), Zn (0.11), and Al (0.13) with the lowest estimated correlations and Cu (0.50), Hg (0.54), and Se (0.66) with the highest. As, Co, Mn, and Pb had correlations at 0.31, 0.32, 0.41, and 0.33, respectively. Conclusion: Findings from this study suggest that a ~25 mg subset can be used for the analysis of most toenail metals with ICP-MS. As, Co, Cu, Hg, Mn, Pb and Se concentrations in toenail clippings can reasonably reflect chronic exposures over multiple years in the Gulf population. Keywords: Metals, Exposure Assessment