You may not be able to make this calculation if you are trying to determine concentrations of an antigen-specific reactive antibody. It's easier if you are determining antigen levels when you can use pure preparations of antigen as your standard. The results from the assay give you a relative amount of reactive antibodies compared to a reference sample, usually pooled serum.

If the kit reports in RU/ml, then the reference standard is likely prepared only for this assay and may not be appropriate for other assays. The supplier will usually make sure that their standard performs equivalently across production lots.

There are also international standards, for which the units are reported as IU/ml. These reference standards can be used to compare assay results in multi-center studies.

Neither of the reference standards are completely defined. That is, the composition of the standard is not known precisely. But the performance is validated to make sure the standard gives reproducible results. Determining the cutoff values for disease vs. healthy is done empirically and cannot be correlated to a precise concentration of a subset of reactive antibodies.

Short answer: You can't make that calculation.

Thank you for your answer and reference.

Thank you for your answer