Twenty-four-hour urinary trace element excretion: reference intervals and interpretive issues

Background: Introduction of inductively coupled plasma mass spectrometry (ICP-MS) into clinical laboratories has led to an increasing application of analyses to risk assessment for toxicity from environmental exposure to trace elements, and in occupational monitoring. Interpretation of results from random urine samples may be problematic and measurement of excretion over 24 h is sometimes preferable. Recent reference data are sparse. Methods: Twenty-four-hour urine samples from 111 healthy adults from the renal stones clinic in Southampton, UK, were analysed for 31 trace elements using ICP-MS and for zinc using atomic absorption spectroscopy. Non-parametric 0.95 coverage intervals were determined for trace element excretion per 24 h and as a ratio to creatinine, for the full study cohort and separately for men (n = 77) and women (n = 34). Results: Beryllium was undetectable in 95% of samples, bismuth in 87% and uranium in 75%. In comparison with published ranges, reference intervals for this cohort were higher for molybdenum, tin and vanadium, and for arsenic due to inclusion of fish arsenicals. Aluminium, chromium, iron, lead and mercury were lower. In our cohort, 24-h excretion of 17 elements was significantly higher in men than in women. However, when expressed as trace element to creatinine ratios, the situation reversed strikingly. Because of their lower creatinine excretion, ratios for 18 elements were significantly higher for women. Conclusions: New adult reference intervals were obtained for 24-h urine trace element excretion. Trace element:creatinine ratios must be used cautiously, with separate ranges for men and women.