Application of benchmark analysis for mixed contaminant exposures: Mutual adjustment of two perfluoroalkylate substances associated with immunotoxicity

Background Developmental exposure to perfluorinated substances is associated with deficient IgG antibody responses to childhood vaccines as an indication of depressed immune system functions. As this outcome may represent a critical effect of these substances, calculation of benchmark dose (BMD) results would be useful for standards setting to protect exposed populations against adverse effects. However, in the mixed exposure setting of most epidemiological evidence, the two major and inter-related substances associated with this adverse effect have shown similar benchmark results that raise concerns about possible confounding. Methods With the aim of better characterizing the immunotoxicity impact of the two major perfluorinated substances in the mixed exposures, we carried out BMD calculations on prospective data from two prospective birth cohort studies from the Faroe Islands with a total of 1,146 children. Exposure data included serum concentrations of perfluorooctane sulfonate and perfluorooctanoate at birth and at age 5 years and, as outcome parameters, the serum concentrations of specific IgG antibodies against tetanus and diphtheria at ages 5 and 7. We calculated the BMDs and their lower confidence bounds (BMDLs) and included mutual adjustment for the two compounds. Results The BMDLs for the two immunotoxicants were of similar magnitude before and after adjustment. Both substances showed lower results for a logarithmic dose-response model, which also provided a slightly better fit than a linear dose model for both antibodies. We also used a broken curve shape that allowed a different slope below the median exposure. Postnatal exposure as represented by the age 5 serum concentration, showed a stronger association with the antibody outcomes than the prenatal exposure. Due to the correlation between the two immunotoxicants, the mutual adjustment resulted in elevated BMD results and p values. However, the BMDL values were virtually unchanged. Conclusions Adjustment for co-exposure to another immunotoxicant increased the variance and the BMD values, but affected the BMDL values only to a negligible extent. These calculations are in accordance with an interpretation that, when two toxicants appear to affect an outcome to an almost equal degree and none of them is known to be solely responsible, the exposures should both be considered responsible and attract equal regulatory attention until further evidence shows otherwise.