Davis NL, et al. - Due to the possible impact of concentration of antiretroviral (ARV) drug in plasma and amounts of drug excreted into breastmilk on HIV viral load and perinatal HIV transmission, researchers analyzed mothers randomized to postpartum maternal ARVs or daily infant nevirapine during 28 weeks of breastfeeding in the Breastfeeding, Antiretrovirals, and Nutrition study. All mothers who transmitted HIV to their infants between 2 and 28 weeks and 15% of mothers who did not (n = 27 and 227, respectively) were included for this cohort study. Lower odds of having detectable HIV RNA and a reduced rate of breastmilk HIV transmission were noted in correlation with having plasma drug concentration above the effective drug concentration (EC50). Lower odds of having detectable maternal HIV RNA was also evident in correlation with having breastmilk drug concentration above the EC50. These findings emphasize the significance of confirming satisfactory drug concentration for viral suppression and preventing breastmilk HIV transmission.