"Following the course of vaccination with the recombinant hepatitis B vaccine at month 7, blood samples were drawn for anti-hepatitis B surface antigen titration using a radioimmunoassay test (AUSAB, Abbott Laboratories, Chicago, IL). Subjects with titers <1 mIU/mL, the assay cut-off, were determined to be seronegative; anti-hepatitis B surface antigen titers 10 mIU/mL were considered to be protective."