Well, there is a calculator. The CPRA estimates the percentage of donors that would be incompatible with the candidate, based on the candidate's unacceptable antigens.
Check out http://www.unos.org/resources/frm_cpra_calculator.asp
When you go to the calculator, you enter all the unacceptable antigens for the patient and it spits out a value.
This has been validated by OPTN/UNOS Histocompatibility subcommittee. Then based on the center, the decision is made. This algorithm is a more accurate measure of sensitization because it accounts for both Class I and II antigens
You can learn more about it at the UNOS website