Comparison of representative ranges based on U.S. patient population and literature reference intervals for urinary trace elements.

Reference intervals for trace elements are very hard to obtain because of the difficulty of defining a nonexposed reference population. However, representative ranges for trace elements obtained from a general patient population can provide useful information in interpreting laboratory results. We have used urine specimens submitted for trace metal analysis from patients residing in the United States to calculate representative ranges for 25 urinary trace elements, and to compare them to reference values taken from the literature. All urine analytes were measured by inductively-coupled plasma-mass spectrometry except chromium, which was measured by graphite furnace atomic absorption spectroscopy. For representative range calculation two approaches were used. In the non-parametric calculation first, the top 10% of results were discarded assuming that those specimens came from individuals with unusually high trace element exposures. Next the central 95% of the remaining data was taken as the reference interval. In the parametric calculation the specimens from exposed or not healthy individuals were assumed to appear as outliers and were discarded. The mean and S.D. were calculated, and used to determine representative ranges. The two approaches yielded very similar results, and worked remarkably well for 14 analytes. There were minor discrepancies for 7 analytes, and major for 4 analytes. All analyses of urinary trace elements included a urine creatinine value, which was used to express urinary trace element concentrations in terms of creatinine ratio. This corrects for differences in urine concentration that affects the results for random specimens.