The high prediction accuracy of the identified biomarkers replicated in an independent cohort of women who were depressed during pregnancy. In this group, the PPD status was segregated with 88% accuracy; however, the prediction was in the opposite direction, driven by differences at the HP1BP3 locus. An analysis of cell subfraction distributions across cohorts identified a difference in the ratio of monocytes to lymphocytes and granulocytes significantly decreased in the depressed cohort that appeared to account for the discrepancy. Our data are consistent with genome-wide expression studies of WBCs taken from women after parturition that demonstrated an association of immune system-related genes with depression scores.45 Incorporation of the DNA methylation biomarkers with cell count data enabled the prediction of PPD status in the entire cohort of 51 women with an AUC of 0.82. A potential confounding factor is that DNA methylation between the prepartum euthymic and depressed cohorts was assessed in two separate batches, as all initial analyses were performed on the euthymic cohort only. To control for this, we normalized DNA methylation levels at all 473 loci used for
