Upon identification of an antibody, the next step is to locate compatible units for your patient. This can be achieved by screening an appropriate number of units using the frequency of the antigen in the population and the number of units needed, with this formula:
(# of units required) / (% antigen negative)
For example, if you needed 6 units negative for the little c antigen, you would need to know that 20% of the population is c-negative to plug the required information into the formula:
6 / 0.20 = 30
With this calculation, it was determined that approximately 30 units will be screened to find 6 that are c-negative.
Finding units negative for multiple antigens uses the same principle. Take the number of units required and divide by the negative frequencies of both antigens being negative in the same donor.
For example, if you needed 2 units negative for little c and K, which have negative frequencies of 20% and 90% respectively, plug those numbers into the formula and perform the calculation:
2 / (0.20 x 0.90) = 11.11
A minimum of 11 units are required to screen to find 2 that are c-negative and K-negative.
