If a patient has multiple antibodies, the number of units that will need to be screened to find units negative for all of the affected antigens can be estimated as follows:
patient with anti-X, anti-Y and anti-Z
frequency of X in the population = 0.x;
(1.00 - 0.x) = frequency of those negative for antigen X.
frequency of Y in the population = 0.y;
(1.00 - 0.y) = frequency of those negative for antigen Y.
frequency of Z in the population = 0.z;
(1.00 - 0.z) = frequency of those negative for antigen Z.
The frequency of the combined phenotype negative for X, Y and Z =
= ((1.00 - 0.x) * (1.00 - 0.y) * (1.00 - 0.z))
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