Temporal stability of urinary cadmium in samples collected several years apart in a population of older persons

Objectives: There is growing evidence that urine cadmium is a temporally stable biomarker indicative of long-term cadmium exposure; however questions remain with regard to generalizability to older persons, the impact of changes in smoking behavior, and the degree of temporal stability when repeat sample collection spans years instead of weeks or months. Methods: Using archived samples from cohorts of older men (Osteoporotic Fractures in Men (MrOS-US)) and women (Study of Osteoporotic Fractures (SOF)) (mean age = 80 at study visit 2), we analyzed two morning urine samples each from 39 men and 18 women with a diverse self-reported smoking history. For MrOS, samples were collected approximately 6 years apart, and 4 years apart for SOF. Intra-class correlations were computed to assess temporal stability, and adjusted for age and body mass index. Results: The median creatinine-adjusted urinary cadmium levels (0.39 mu g/g for men, 0.89 mu g/g for women) were similar to levels expected for these age/sex groups in the US according to the National Health and Nutrition Examination Survey. The overall intra-class correlation was high (ICC = 0.85; 95% CI: 0.76-0.91) and similar between cohorts (MrOS: ICC = 0.74; 95% CI: 0.58-0.86; SOF: ICC = 0.81; 95% CI: 0.59-0.93), but slightly lower among those who stopped smoking between visits of sample collection (ICC = 0.64; 95% CI: 0.31-0.87) or among former smokers who quit prior to the first sample collection (ICC = 0.68; 95% CI: 0.25-0.93). Conclusions: We report good-to-excellent reproducibility of urine cadmium using morning urine samples collected 4-6 years apart from older men and women, but slightly lower correlations among those with a history of smoking. Single measures of urine cadmium are a reliable biomarker in older men and women.