BMC Public Health 2008, 8:72 http://www.biomedcentral.com/1471-2458/8/72 ng/mL for testosterone, 5 pg/mL for estradiol, 0.33 ng/mL for AAG, and 3 nmol/L for SHBG. The coefficients of var- iation (CV%) for quality control specimens included dur- ing the analyses of the NHANES III samples ranged from 5.8 to 5.9% for testosterone, 2.5 to 6.7% for estradiol, 5.0 to 9.5% for AAG, and 5.3 to 5.9% for SHBG. Free testo- sterone concentrations were estimated from measured tes- tosterone, SHBG, and albumin, while free estradiol concentrations were estimated from measured estradiol, SHBG, and albumin [27,28]. Statistical Analysis Participants were categorized into quartiles of urinary cad- mium based on the weighted population distribution. Age and race-ethnicity adjusted means and percentages were calculated by linear regression for continuous covari- ates and logistic regression for dichotomous covariates. Due to skewed distributions of sex steroid hormones, we calculated adjusted geometric means by quartile of cad- mium using multiple linear regression on log-trans- formed hormone levels. We also present analyses for molar ratios of estradiol to testosterone, testosterone to SHBG, and estradiol to SHBG. Initial models adjusted for age and race-ethnicity. Subsequent models further adjusted for smoking status and serum cotinine. Addi- tional models further adjusted for household income, physical activity, alcohol consumption, percent body fat, blood lead, and serum selenium. Tests for linear trend across quartiles of cadmium were computed by including an ordinal variable with the median of each quartile of cadmium in the linear regression models. Due to the importance of smoking in determining cadmium levels and the possibility of overadjustment, we additionally present models separately for never, former, and current smokers. Data were analyzed using SUDAAN (version 9.0; Research Triangle Institute, Research Triangle Park, NC) to account for the complex NHANES sampling design, including unequal probabilities of selection, over-sampling, and non-response. Results The median level (range) of urinary cadmium in the study sample was 0.34 µg/g creatinine (0.003–4.22 µg/g). Par- ticipants with higher cadmium levels were more likely to be older, to be non-Hispanic black, to be current smokers, to have household incomes < $20,000, not to exercise, and to have higher blood lead levels (Table 1). The median levels (range) of urinary cadmium among never (n = 436), former (n = 416), and current (n = 410) smok- ers were 0.21 µg/g (0.003–4.22 µg/g), 0.42 µg/g (0.004– 2.32 µg/g), and 0.56 µg/g (0.004–4.02 µg/g), respectively. After adjustment for age and race-ethnicity, higher cad- mium levels were associated with higher levels of total tes- tosterone, total estradiol, SHBG, estimated free testosterone, and estimated free estradiol (each p-trend < 0.05), but not with AAG or any of the molar ratios (Table 2). After additionally adjusting for smoking status and serum cotinine, the associations of urinary cadmium lev- els with sex steroid hormones and their molar ratios were small and not statistically significant (each p-trend > 0.05). After stratifying by smoking status, the associations of urinary cadmium levels with sex steroid hormones and their molar ratios were small and not statistically signifi- cant for never, former, and current smokers (each p-trend > 0.05; Table 3). However, among never smokers there was a marginally significant, negative association between urinary cadmium and total testosterone (p-trend = 0.06) and among current smokers there was a marginally signif- Table 1: Age and race-ethnicity adjusted participant characteristics* by quartile of creatinine corrected urinary cadmium Quartile 1 < 0.18 µg/g Quartile 2 0.18–0.33 µg/g Quartile 3 0.34–0.62 µg/g Quartile 4 ≥ 0.63 µg/g p-trend Age, years Non-Hispanic white, % Non-Hispanic black, % Mexican-American, % Current smokers, % Former smokers, % Serum cotinine, ng/mL Low income, % Consume alcohol, % No exercise, % Percent body fat, mean percent Blood lead, µg/dL† Serum selenium, ng/mL *Mean or percentage (standard error) †Geometric mean (95% confidence interval) 31.7 (0.8) 83.9 (2.9) 6.5 (1.5) 4.2 (1.0) 11.2 (2.1) 29.3 (4.3) 26.4 (8.4) 24.1 (3.0) 75.6 (4.1) 40.6 (6.1) 25.8 (0.5) 3.2 (2.8, 3.6) 126.5 (2.7) 37.1 (0.7) 80.3 (3.4) 10.9 (2.2) 5.6 (1.0) 20.4 (3.7) 33.9 (4.2) 39.6 (8.4) 20.4 (2.7) 71.5 (3.4) 30.1 (3.5) 26.1 (0.5) 3.6 (3.2, 4.0) 126.7 (1.1) 44.6 (1.3) 74.5 (5.6) 8.1 (1.1) 4.5 (0.9) 42.9 (4.2) 35.8 (3.7) 107.2 (12.9) 31.0 (2.9) 74.8 (3.2) 41.0 (4.4) 25.4 (0.6) 4.3 (3.9, 4.8) 127.3 (1.3) 54.3 (1.2) 74.3 (5.2) 11.1 (1.7) 4.7 (0.8) 73.6 (2.4) 25.7 (3.2) 231.0 (12.1) 42.1 (4.0) 72.7 (3.8) 55.2 (4.2) 24.7 (0.5) 5.5 (4.9, 6.3) 121.7 (1.3) < 0.001 0.07 0.03 0.99 < 0.001 0.06 < 0.001 < 0.001 0.76 0.005 0.05 < 0.001 0.05 Page 3 of 7 (page number not for citation purposes)