Shankar et al. those who had smoked > 100 cigarettes in their lifetimes were considered former smok- ers if they answered negatively to the question “Do you smoke now?” and current smokers if they answered affirmatively. Rigorous procedures with quality control checks were used in blood collection, and details about these procedures are provided in the NHANES laboratory/medical tech- nologists procedures manual (NCHS 2010d). We used the modified hexokinase method to measure serum glucose levels at the University of Missouri Diabetes Diagnostic Laboratory (Columbia, MO, USA). Diabetes mellitus was defined, based on the recent guidelines of the American Diabetes Association (2011), as a serum glucose level of > 126 mg/dL after fast- ing ≥ 8 hr or of > 200 mg/dL for those who fasted < 8 hr before their NHANES visit, a glycosylated hemoglobin value of > 6.5%, or a self-reported current use of oral hypoglycemic medication or insulin. Seated SBP and DBP were measured using a mercury sphygmoma- nometer according to the American Heart Association and Seventh Joint National Committee recommendations (Chobanian 2003), and up to three measurements were averaged. Patients were considered hyper- tensive if they reported current use of blood pressure–reducing medication or had SBPs of > 140 mmHg or DBPs of > 90 mmHg (NCHS 2010b). In NHANES 2003–2004, serum creati- nine measurements were conducted at the Collaborative Laboratory Services (Ottumwa, IA, USA) using the Beckman Coulter Synchron LX20 chemistry analyzer (Beckman Coulter, Fullerton, CA, USA) using the Jaffe rate method (kinetic alkaline picrate). Coefficients of variation ranged from 1.5% to 4.3% (NCHS 2010c). In a calibration sub- study, serum creatinine assays were performed on 190 stored specimens from NHANES 2003–2004 at the Cleveland Clinic labora- tory (Cleveland, OH, USA) using the Roche coupled enzymatic assay that was traceable to gold-standard reference methods, including an isotope dilution mass spectrometric method for serum creatinine using standard references methods [NIST standard reference material (SRM) 967; National Institute of Standards and Technology, Gaithersburg, MD, USA] and confirmed by analysis of CAP LN-24 (College of American Pathologists Creatinine Accuracy Calibration Verification/Linearity Survey) linearity set based on NIST-assigned values (Selvin et al. 2007). There were no sig- nificant differences in results between these two measurements, and therefore it was con- cluded that there was no correction neces- sary for serum creatinine values in NHANES 2003–2004 (Selvin et al. 2007). The glom- erular filtration rate was estimated (eGFR) from serum creatinine using the four-variable 1298 Modification of Diet in Renal Disease study equation (Levey et al. 2006) as follows: eGFR = 175 × (serum creatinine in mg/dL)–1.154 × (age in years)–0.203 × (0.742 if female) × (1.21 if black). Chronic kidney disease was defined as an eGFR of < 60 mL/min/1.73 m2, consistent with definitions of the National Kidney Foundation Kidney Disease Outcome Quality Initiative (KDOQI; National Kidney Foundation 2002) working group and the Kidney Disease Improving Global Outcomes (KDIGO) (Levey et al. 2006). Previous measures of BPA in biologi- cal matrixes involved techniques such as gas chromatography or high performance liquid chromatography (Ye et al. 2005). To achieve enhanced sensitivity and selectivity over pre- vious methods, in the current NHANES, measures of environmental phenols were derivatized to alkyl or acyl derivatives before gas chromatography–mass spectrometry analy- sis (NCHS 2010a). The lower limit of detec- tion for BPA concentrations was 0.36 ng/mL. Main outcome of interest: PAD. For study subjects with at least one arm and weigh- ing ≤ 400 lb, supine SBP was measured with blood pressure cuffs on the right arm, compressing the brachial artery and the two posterior tibial arteries (NCHS 2010a). For subjects 40–59 years of age, two measure- ments were taken at each site and averaged, and for patients ≥ 60 years of age, one measure was taken at each site. For subjects with condi- tions precluding the measurement of the right arm, left brachial artery SBP was taken. Left and right ABI values were calculated as the ratio of left and right ankle SBP, respectively, to arm SBP. The smallest of the left and right ABI measurements was used. Patients with ABIs ≥ 1.5 (n = 3) are expected to have severe arterial rigidity and were excluded from the present analyses (Newman et al. 1993). For these analyses, PAD was defined as ABI < 0.9 (American Diabetes Association 2011; Selvin and Erlinger 2004). Because of the selection criteria for ABI measurements in NHANES (subjects ≥ 40 years of age), subjects who had ABI measurements taken were significantly older than the general NHANES cohort but were otherwise similar with respect to demo- graphic factors (data not shown). Statistical analysis. Urinary BPA was categorized into tertiles (< 1.4 ng/mL, 1.4–3.5 ng/mL, > 3.5 ng/mL). We hypothe- sized that high BPA levels are associated with PAD. Odds ratios (ORs) and 95% confidence intervals (CIs) for PAD in association with BPA were calculated by taking the lowest tertile (tertile 1) as the referent, using multi- variable logistic regression models. We used two models: one adjusted only for age and sex, and a multivariable model that also adjusted for race/ethnicity (non-Hispanic whites, non- Hispanic blacks, Mexican Americans, others), education (< high school, high school, > high school), household income (< $25,000, $25,000–54,999, ≥ $55,000), smoking status (never, former, current), pack-years of smok- ing, alcohol intake (nondrinker, moderate drinker, heavy drinker), BMI (normal, over- weight, obese), hypertension (present, absent), diabetes (present, absent), urinary creatinine (milligrams per deciliter), eGFR (milliliters per minute per 1.73 m2), and total cholesterol (milligrams per deciliter). Trends in the OR of PAD across increasing urinary BPA categories were determined by modeling BPA as an ordi- nal variable. We examined effect modification by performing stratified analysis by categories of sex, race/ethnicity, BMI, diabetes mellitus, and hypertension. We also conducted formal statistical tests for interaction by including multiplicative cross-product interaction terms in the multivariable logistic regression models; p < 0.1 was interpreted as a statistically signifi- cant interaction. Sample weights that account for the unequal probabilities of selection, over- sampling, and non-response were applied for all analyses using SAS (version 9.2; SAS Institute Inc., Cary, NC, USA) and SUDAAN soft- ware (version 10; RTI International, Research Triangle Park, NC, USA); SEs were estimated using the Taylor series linearization method. Results The median (interquartile range) of uri- nary BPA levels was 2.30 (3.60) ng/mL; the 25th, 50th, and 75th percentile values were 1.00, 2.30, and 4.60 ng/mL, respectively. Participants with higher levels of BPA were more likely to be men, non-Hispanic blacks, and former or current smokers and had higher levels of urinary creatinine. Table 1 shows these characteristics for the study popula- tion by teriles of urinary BPA levels. Overall, we observed a positive association between increasing BPA levels and PAD on the basis of the age- and sex-adjusted model and the multivariable-adjusted model, with significant (p < 0.05) trend tests (Table 2). This positive association persisted when BPA was analyzed as a continuous variable (1-SD increase in log- transformed BPA). In a supplementary analysis, we examined the association between BPA levels and PAD after excluding n = 10 subjects with very high BPA values (levels > 30 ng/mL). The results were found to be essentially similar. The multi- variable OR of PAD associated with 1 SD of log-transformed BPA was 1.60 (1.16–2.21). We also examined the association between increasing tertiles of BPA and self-reported CVD, and the magnitude of association was found to be weaker than that for PAD. ovlume 120 | number 9 | September 2012 • Environmental Health Perspectives