I conquer that NPU should adopt the unconventional unit ”BAU”. The term “binding” belongs to the component; SARS CoV 2 (Spike glycoprotein) binding antibody(IgG) and the unit is an International Unit as defined by WHO. As NIBSC describes the unitage per mL, the unit as usual for International Units from WHO also has to be multiplied by a factor 10^3.
Best regards
Gunnar