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Context.--Clinical testing for Wilson disease (WD) is potentially challenging. Measuring the fraction of labile bound copper (LBC) to total copper may be a promising alternative diagnostic tool with better sensitivity and speci- ficity than some current biomarker approaches. A dual filtration--based inductively coupled mass spectrometry (ICPMS) assay to measure LBC in serum was developed. Objective.--To establish a reference interval for LBC and LBC to total copper (LBC fraction) in a healthy adult population, and to examine associations between total copper, LBC, and LBC fraction with age, sex, menopausal status, hormone replacement therapy, and supplement use. Design.--Serum samples were collected from healthy male (n = 110) and female (n = 104) patients between the ages of 19 and 80 years. Total copper and LBC were analyzed using ICP-MS. Results were used to calculate the LBC fraction. Reference intervals were calculated for the 2.5th and 97.5th percentiles for both LBC and LBC fraction. Results.--The reference intervals for LBC were determined to be 13 to 105 ng/mL and 12 to 107 ng/mL for female and male patients, respectively. The reference intervals for the LBC fraction were 1.0% to 8.1% and 1.2% to 10.5% for female and male patients, respectively. No significant associations were found regarding age, menopausal status, hormone replacement therapy, or vitamin and supplement use. Conclusions.--Sex-specific reference intervals have now been established for LBC and LBC fraction. These data in conjunction with further testing of WD populations can be used to assess the sensitivity and specificity of LBC fraction in screening, monitoring, and diagnosis. [ABSTRACT FROM AUTHOR]