Breast Cancer Res Treat (2012) 136:267–275 3 was 0.01–0.03 ng/mL for tetra- through hepta-chlorobi- phenyls, using 1–1.5 mL plasma. As described previously [23], we used all observed positive values of PCBs in analyses, even those reported to be below the limit of detection. We were able to assay archived serum samples for the PCB congeners which are shown in Table 1. These cong- eners include those most commonly reported in prior studies, comprised the congeners with highest concentra- tions, or grouped by potential biological activity as previ- ously recommended [24]. We randomly assigned the order of samples within and across batches and analyzed case– control pairs in the same batches to minimize differences due to laboratory drift. The laboratory was blind as to case or control status of the samples. Intra-batch coefﬁcients of variation ranged from 5 % for PCB 180 to 18 % for PCB 101. Total cholesterol and total triglycerides were mea- sured enzymatically on the Hitachi 911 analyzer (Roche Diagnostics, Indianapolis, IN) in a lab certiﬁed by the Centers for Disease Control and Prevention and the Table 1 Distribution of PCBs in breast cancer controls and cases 269 National Heart Lung and Blood Institute Lipid Standardi- zation Program. Statistical analysis This report is based on 112 case–control pairs, matched on year of birth, after excluding 2 pairs with insufﬁcient serum for lipid assays, 2 pairs with missing data on body mass index, and 17 pairs where one or more member of the pair was missing information on PCB 167, which was not quantiﬁed in one of the assay batches. In a sensitivity analysis, we imputed missing PCB 167 by matching on case status and date of blood draw and assigning a PCB value for the missing individual based on a random pick of all matches. Results were similar (associations were of a similar magnitude with overlapping 95 % conﬁdence intervals and all associations remained statistically signif- icant, deﬁned as p \ 0.05) whether or not the individuals with imputed values were included. We present ﬁndings only for individuals with non-missing data on all study Classiﬁcation [24] Estrogenic Anti-estrogenic A. Non-ortho, mono-ortho, dioxin-like B. Di-ortho, limited dioxin activity Phenobarbital, CYP1A, and CYP2B inducers PCB 101 187 201 66 74 105 118 156 167 138 170 99 153 180 183 203 203/167 ? 187 Controls (N = 117) Cases (N = 123) Percentile (mmol/l) Percentile (mmol/l) 25th 0.37 0.38 0.09 0.89 0.62 0.25 1.19 0.11 0.08 1.58 0.30 0.28 1.88 0.89 0.13 0.21 0.34 50th 0.74 0.48 0.23 1.34 0.89 0.40 1.62 0.28 0.19 2.05 0.46 0.58 2.38 1.21 0.23 0.30 0.43 75th 1.07 0.66 0.33 1.78 1.23 0.64 2.02 0.42 0.30 2.55 0.66 0.80 3.08 1.54 0.35 0.42 0.52 25th 0.34 0.38 0.14 0.96 0.62 0.25 1.19 0.11 0.08 1.50 0.35 0.25 1.86 0.94 0.10 0.23 0.38 50th 0.55 0.48 0.23 1.47 0.86 0.43 1.53 0.28 0.14 2.02 0.48 0.52 2.43 1.21 0.23 0.35 0.50 75th 0.92 0.63 0.30 1.95 1.27 0.61 2.11 0.39 0.25 2.77 0.63 0.86 3.08 1.57 0.33 0.47 0.63 -0.06 -0.07 -0.01 0.10 0.01 -0.01 -0.03 -0.03 -0.03 -0.09 -0.01 -0.04 -0.08 0.01 -0.03 0.02 0.09 Difference within matched pairs (case–control, N = 112) Mean (mmol/l) p value Percent [ LOD 0.19 0.30 0.71 0.17 0.54 0.92 0.94 0.68 0.12 0.56 0.74 0.30 0.40 0.63 0.32 0.20 89 100 71 99 95 81 100 64 49 100 97 83 100 100 66 90 123 Variable number of subjects in each group is due to missing data on one or more of the PCB congeners shown. The PCB score (PCB 203/(PCB 187 ? PCB 167)) was based on the best ﬁtting model as described in the text, and is provided to describe the distribution of the mixture of signiﬁcant PCB predictors. p values are not presented for the PCB score because the score was created after analysis, as described in text. p values shown are for Wilcoxon Signed Rank Test of differences (case–control) within age-matched case–control pairs LOD limit of detection