F2 and diphtheria: Using dilution curves of the international standard preparations, values of diphtheria-, tetanus- and pertussis-specific antibody concentrations of the in-house reference serum were determined: DT: 3.0 IU/mL; TT: 3.5 IU/mL; PT: 97 IU/mL; FHA: 292 IU/mL; and Prn: 1333 IU/mL.