Factors influencing urinary cadmium as a biomarker of human cadmium exposure

Background  Health risk assessment of cadmium exposure usually uses urinary cadmium as an exposure indicator, but some new studies show disagreement on urinary cadmium as an exposure biomarker.

Objective  This cross-sectional study aims to assess the factors influencing urinary cadmium as a biomarker of human cadmium exposure.

Methods  Using a cross-sectional study design, we selected residents of a city in eastern China with no known cadmium contamination as study population. A total of 1 235 local residents participated in the study and were classified by age into preteenagers (< 12 years), teenagers (12-17 years), and adults (≥ 18 years) three groups. The peripheral blood and urine samples of the participants were collected, and cadmium in the biological samples was determined by inductively coupled plasma mass spectrometry (ICP-MS). The median (M) and the 25th and 75th percentiles (P₂₅, P₇₅) were used to represent the distributions of urinary cadmium, blood cadmium, and urinary creatinine. A natural cubic spline analysis was used to describe the changes of urinary cadmium, blood cadmium, and urinary creatinine with age in 997 current non-smoking participants. The correlations of urinary cadmium with sex, age, blood cadmium, urinary creatinine, and body mass index (BMI) were further assessed using multiple linear regression models.

Results  The urinary cadmium exposure levels in the selected residents ranged from 0.14 to 0.37 μg·L^-1. The teenagers' median (P₂₅, P₇₅) urinary creatinine 1.31(0.89, 1.80)g·L^-1 was higher than preteenagers's 0.78(0.55, 1.06)g·L^-1 and adults' 0.94(0.62, 1.33)g·L^-1 (P < 0.01). The un-adjusted urinary cadmium increased with higher age in both non-smoking men and women. In men, the urinary cadmium peaked at about 60 years of age and then declined. In women, however, the urinary cadmium increased gradually before age 50, then appeared stable. In both sexes, the urinary creatinine increased during preteenage period, peaked during teenage period, then declined steadily with age, and increased again after age 70. The results of multiple linear regression analysis showed that the urinary cadmium without creatinine correction was associated with blood cadmium and urinary creatinine in all age groups. In adults, the influencing factors also included age and sex, while sex was no longer statistically significant after smoking stratification. The urinary cadmium adjusted for creatinine was associated with blood cadmium and urinary creatinine in teenagers and adults, but not in preteenagers. The urinary cadmium levels in current smokers un-ajusted: 0.44(0.40, 0.49)μg·L^-1; adjusted: 0.42(0.38, 0.46)μg·g^-1 creatinine were higher than those in neversmokers un-ajusted: 0.31(0.29, 0.33)μg·L^-1; adjusted: 0.36(0.33, 0.38)μg·g^-1 creatinine (P < 0.01).

Conclusion  Urinary cadmium is associated with blood cadmium, urinary creatinine, age, and smoking, and these factors need proper correction when urinary cadmium is used as a biomarker for cadmium exposure.