How can the other possible antibodies be excluded (phenotype of red cells and number needed)?

To exclude anti-C in a patient with a known anti-D requires two r'r (Cde/cde) cells or one cell from a D-negative donor who is homozygous for C (Cde/Cde).

Similarly, to exclude anti-E in a patient with a known anti-D requires two r''r (cdE/cde) cells or one cell from a D-negative donor who is homozygous for E (cdE/cdE).

Cells from D– CC and D– EE donors are extremely rare and almost always unavailable.