Plasma aflatoxin B1-lysine (AFB1-lys) was measured at the Department of Environmental Health and Engineering of the Johns Hopkins Bloomberg School of Public Health, using a minor variation of the method reported by McCoy et al [18 (link)]. Serum (150 μL) was spiked with an internal standard (0.5 ng of AFB1-d4-lysine in 100 μL), combined with Pronase (EMD Millipore, Billerica, MA) protease solution (3.25 mg in 0.5 mL phosphate-buffered saline), and incubated for 18 hours at 37°C. Solid-phase extraction-processed samples (Oasis MAX columns; Waters, Milford, MA) were analyzed with ultrahigh pressure liquid chromatography (UHPLC)-isotope dilution mass spectrometry on a Thermo Fisher Scientific (San Jose, CA) system composed of a Vanquish UHPLC and a TSQ Quantis triple quadrupole mass spectrometer in positive electrospray ionization mode [19 (link), 20 (link)]. The limit of quantification (<20% coefficient of variation) was 14 pg AFB1-lys/mL serum.